Ondina is a mermaid from the Mako Island pod who decides that she and Mimmi must go against the mermaid council's rules and runs away to drain Zac of his powers. She is determined to save Mako for the pod, which holds deep sentimental value to her. Like most mermaids, she holds deep resentment for land people and mermen. When she and Mimmi are introduced to Zac, she quickly grows blatantly hostile towards him when he is slow to comply with their requests and even goes so far as to engage him in a brief fight. Her treatment of Evie is hardly any better when she and Mimmi accidentally cause her to be transformed into a mermaid after failing to remove Zac's powers.
She only agrees to try to change her back when Sirena threatens to rat them out to the mermaid council. Her prejudice towards land people nearly causes them to be exposed when she refuses to stop their first attempt to change Evie back despite Zac warning her that their plan is not working and putting Evie on the brink of being exposed. She also refuses Evie's request to collaborate with her in the future until she threatens to blow her secret. When the group discovers a chamber on Mako built by mermen that causes her to disappear, she grows more and more fearful to what might happen if Zac tries to activate it. Her overbearing nature and close minded attitude would later cause her to become Zac's first suspect when someone threatens Evie's dad's diving business.
She is later proven innocent which shows that while prejudicial, she would not go so far to causing damage or hurting people. In time, she gradually begins to see that Zac is not as much of a threat as the pod made him out to be. After Zac attempts to apologize for wrongly accusing her by humorously (and rather badly) attempting to do it in whale language, Ondina begins to see what a likeable guy Zac really is. She accepts his apology and they become friends. Although she still believes at first that she must find a way to sever Zac's connection to Mako so the pod will return, she soon realizes that between Evie becoming a mermaid and later meeting another merman named Erik, her mission to save Mako for the pod is becoming much more complicated then she originally thought. However, when Ondina later owns up to the head of the mermaid council Veridia for leaving without permission and she allows her to continue trying to save Mako, she does not reveal Evie's transformation and Erik's arrival, letting her believe that Zac is the only thing standing in the pods way.
Ondina is forced to rethink her prejudice even more when she finds a love interest in Erik. Thinking he is merely a land boy, Ondina hardly thinks anything of him at first although Erik shows interest in her from the start even though Ondina refused to give him the time of day at first. When she accidentally blows her secret to Erik, he reveals who he really is to demonstrate his trustworthiness which causes her to return his feelings. Throughout the season, they continue to grow closer to each other. This would change briefly when Erik lies to her about agreeing to keep Zac away from the chamber. After learning of this deception, she along with Mimmi and Veridia interrupt their attempt to start the chamber. In the ensuing confrontation, Rita reveals that Zac is actually a real merman and Mimmi's brother. When Mimmi resolves to protect her brother from Veridia and she orders Ondina to reason with her, Ondina refuses knowing it would be wrong to try to force her best friend to turn her back on her own flesh and blood. After briefly apologizing to Veridia, Ondina turns against her and joins Mimmi in defending Zac. This act of defiance would cost her and Mimmi their place in the pod and seemingly any chance they had of saving Mako. When an overwhelmed Zac refuses to accept Mimmi as his sister and briefly makes the decision to sever all further connections with the the girls, Ondina is disappointed by this and attempts to reason with him by reminding him all that she and Mimmi sacrificed to protect him from harm confirming that Veridia indeed would have killed him had she, Mimmi, and Rita not interfered. However, Zac would hear nothing of it initially. When Mimmi expresses confidence that Zac will come around, Ondina expresses uncertainty she's right but at the same time is hopeful that she is which eventually does prove true. However, Ondina is furious with Erik for lying to her, but she eventually forgives him.
Throughout the second half of season 2, Ondina and the others continue to work for their goal of reclaiming Mako for the pod. She also has fully accepted the futility of her original mission to remove Zac's powers and how important of a relationship Mimmi now has with him, realizing that it is better to work with him instead of against him and is noticeably more respectful to him as well as Evie. However, Ondina is still against Zac starting up the chamber and is frustrated when Mimmi sides with him despite Ondina's pleas that Mimmi reason with him. Although Mimm tries to insist that Zac promised they would be safe and she intends to go with him to make sure nothing bad happens, Ondina is unsatisfied and leaves in a huff. She later runs into Veridia at the moon pool who has come up with a plan to destroy the chamber by having a mermaid turn the chambers power back on itself with their powers. She enlists Ondina to carry out this task, agreeing to lift hers and the others banishment if she succeeds. Ondina then tries to convince Mimmi not to go with Zac to the chamber, fearing that Mimmi will side with Zac over her like she did previously and try to stop her from destroying the chamber. Mimmi refuses this, unwilling to risk her brother getting hurt, much to Ondina's frustration. Veridia's plan is dealt a further blow when Ondina finds out from Erik that Zac has changed his mind about starting up the chamber. Ondina later meets with Zac and tries to talk him back into learning the chamber's purpose, but he is initially skeptical due to learning from Rita that his ancestor built the chamber and chose not to activate after falling in love with a mermaid. She is eventually able to convince him that doing so is what is best for Mimmi because finding out what the chamber does is the only way the pod will be able to come home and give Mimmi her old life back. This also seemingly provides the perfect opportunity for Ondina to ensure that no one else interferes with her plans. On the night of the full moon, Ondina signals for Zac head to Mako and then secretly follows him as he prepares to activate the chamber. As she prepares to carry out Veridia's plan, the others intervene and warn her that destroying the chamber may destroy the island along with it. They also find out the trident is needed to start the chamber and since it is already destroyed, it seems that it does not matter anyway. After sharing this with Veridia, she lifts the girls banishment and proudly declares that the pod will return by the next full moon much to their delight.
Ondina also continues to grow closer to Erik and they eventually become a real couple. As they spend increasingly more time together, Ondina begins to see the joys of living on land and her resentment for land people and mermen all but vanishes. However, her relationship with Erik causes a rift between her and Mimmi when she starts feeling neglected. Although they do make up, this would later escalate when Erik mentions that the girls may have to choose between land and sea. Torn between Erik and the pod, she seeks advice from Rita who also had to choose between her lover and the pod and she tells her to follow her heart. Much to Ondina's chargin, Mimmi accuses him of stealing a pendant of Rita's to make money to throw a party he planned for Ondina. This causes her to come to the conclusion that Erik was right, and she ultimately decides to give up on the pod for a life with Erik. A heartbroken Mimmi then refuses to speak with Ondina, but Sirena convinces them they although things would be different, they can still be friends. The girls then decide that they must find a way to convince the pod that land people and mermen can be trusted so they can be with all the people they care about.
Later, Ondina's fears of the chamber's dangers are proven true when they discover that the stone that was the tridents source of power is the key to starting the chamber and that it amplifies the stone's already dangerous effects on mermaids. Ondina remains unaware of Erik's hostile actions towards her friends as he seeks the trident stone until he succeeds in getting it. Heartbroken at being betrayed again, she gives him one last chance to give up on the chamber, but he refuses. She and the other mermaids attempt to destroy the stone when Erik starts the chamber, but this fails and it slowly begins to kill them but Zac is able to stop the chamber. Like the other mermaids, Ondina is horrified when it appears that Zac has died in his efforts to stop the chamber and she and Sirena look on in great anguish as Evie and Mimmi weep over his death. She is greatly relived when Mimmi is able to heal him, even starting to choke up at seeing him alive and well again. Afterwards, Ondina ends her relationship with Erik for good, unwilling to risk being his second choice again. Deeply hurting from the end of their relationship and afraid she might change her mind, Ondina completely ignores Erik's final feeble attempt to call out to her and take him back and just keeps on walking without turning back as Erik runs out to the sea in heartbreak never to be seen again. Despite her broken romance with Erik, Ondina is overjoyed that she has succeeded in making Mako safe for mermaids again and at the same time can still take advantage of her new found appreciation for the world of land people. After being praised by Veridia, she and the others head to Mako to greet the pod as they return to Mako at last.
In season 3, Ondina returns to her mermaid life with Mimmi. They are shown to have grown particularly close to Evie, who now hangs out with them on a frequent basis since getting her moon ring. Ondina and Mimmi also make frequent visits on land to hang out at the cafe and Rita's grotto with their other new friends on land such as Zac, Carly, Cam, and David. In this season, Ondina is given a great deal of responsibility when Veridia offers her a position as a teacher for younger mermaids of the pod. Although hesitant at first, Veridia offers encouragement having been won over by her actions in the previous season and Ondina accepts the job. During her first class, one of Ondina's new students wonders off and gets caught in a net. As Ondina prepares to rescue her, she notices a new mermaid named Weilan do it instead. Ondina and Mimmi are later introduced to Weilan by Rita, who she had met in a previous visit to her home in China. Although Mimmi does everything she can to make her feel welcome, Ondina harbors an immediate disliking towards her out of jealousy for saving her student, her personality being similar to her own, and her dry sense of humor that makes her seem difficult to please. She also resents her being different from what Ondina thinks is a real mermaid such as having a phone, sleeping on land, and not having a pod. When Ondina, Mimmi and Evie prepare to take part in the first full moon ceremony since the pod's return, they are suddenly approached by a 60 ft water dragon. After a futile attempt to fight it off with their moon rings, Zac and Weilan arrive and they soon retreat to warn the pod. As the group discusses with Rita and Veridia what they saw, Weilan reveals that the dragon wiped out the Eastern pod and believes no mermaid can stop it. Rita also reveals that the reason Weilan came to Australia was because her grandmother sent her to keep her safe from the dragon. Following this, tension between Ondina and Weilan only grows as Ondina now blames Weilan for bringing the dragon to the pod. She even goes so far as to insult Weilan by saying she is not a real mermaid which greatly angers and offends her. However, after Weilan saves Ondina from being exposed by workers from the marine park who were investigating the moon pool and creates a water spout to prevent them from making future trips to Mako and discovering the pod, the two of them make peace and start to become friends although they are not above wise cracking each other when the opportunity presents itself. Ondina is also overwhelmed with jealousy when Weilan demonstrates a powerful eastern spell that no southern mermaid has ever mastered known as Turn the Tide that reflects any magic aimed at the spell caster back to where it came from.
In addition to teaching the younger mermaids how to use their powers, Ondina also takes Evie under her wing to teach her how to use her moon ring. Some of these lessons go awry such as in "Mopping Up", where Evie causes a mop to develop a mind of it's own with her moon ring and in "New Beginnings" where Evie accidentally causes Ondina and Weilan to be trapped in midair during another lesson. In the process, Ondina and Weilan's respect for one another increases as they work together to get themselves on the ground again. In time however, Evie is gradually able to learn to use her moon ring with Ondina's guidance.
As Ondina and Mimmi prepare to face the dragon, they are both frustrated when Veridia instead tasks them with looking after the younger mermaids instead of fighting with the pod. The two of them both decide to defy Veridia's orders and join the pod anyway. However, Ondina remains unaware that Weilan has secretly made a plan with Zac to have him face the dragon after teaching him Turn the Tide in the hopes that turning its own fire breath will destroy it. She and the rest of the pod are led astray by Weilan to hide on the opposite side of the ocean where the dragon will appear so Zac can carry out their plan. As the pod prepare to fight the dragon, Ondina and Mimmi soon begin to suspect something is wrong when the dragon does not appear to them despite the moon haven already risen. Ondina's suspicions grow when Veridia informs her that their plan is based off of Weilan's instructions.
When Mimmi suddenly has a vision showing her that Zac is facing the dragon in the moon pool, Ondina realizes that Weilan deceived them so Zac could fight it alone. Knowing he will he need help, Ondina and Mimmi race to the moon pool to intervene. When they arrive they find Zac and Evie both uninjured, but Ondina is horrified upon noticing that Evie has lost her tail and powers. Blaming Weilan for their failure to destroy the dragon and Evie losing her tail, Ondina furiously berates her, leaving her feeling incredibly guilty and hurt. She later goes on to tell Mimmi that she thinks Weilan should just leave, which Weilan overhears making her feel even worse. Ondina is also greatly depressed over Evie losing her tail and that all the progress she made learning to use her moon ring was for nothing. She then comes up with an idea to help Evie get her tail back with the 50 moon spell they previously tried on Zac that caused Evie to get her powers in the first place. The spell is unsuccessful ad backfires on Ondina, causing her to become stuck in mermaid form even when she's dry. Now finding herself in Weilan's situation where she caused trouble after trying to help, Ondina realizes she was to harsh with Weilan. After apologizing to Weilan, the two reconcile and Weilan helps reverse the spell.
During the second half of season 3, Ondina is later horrified to learn that Mimmi has agreed to go on a date with Chris, her view of relationships having been tarnished since her experience with Erik. During their date, Ondina watches them obsessively and ruins the date by summoning a rain storm forcing Mimmi to flee. Although Weilan confronts Ondina on what she did, Ondina does not regret her actions at first insisting Mimmi is better off without him. Ondina then tearfully reveals to Weilan that she did what she did because her own relationship with Erik ended in heartbreak and she does not want to see Mimmi suffer the same way, showing that she still hurts from their breakup. After Weilan offers her comfort, she convinces Ondina that Mimmi needs to be free to make her own choices. After admitting what she did to Mimmi and apologizing, Ondina also thanks Weilan for helping let go of her own past. The two then work together to bring Mimmi and Chris back together. Ondina soon learns to be happy about Mimmi's new relationship with Chris, playfully giving him the nickname "Dolphin Boy".
Close to the season climax, the mermaids find a potential solution to defeat the dragon. While attending a book signing of a world famous treasure hunter named Rikki Chadwick, they notice a dragon bracelet that is associated with an ancient Chinese legend known as the Jao Long Dragon. Believing it can slay the dragon, the mermaids ask Rikki for the bracelet, but she turns them away. The girls then enlist Zac's help to steal it, but the heist is ultimately unsuccessful. Ondina is later surprised when Mimmi and Zac have a vision of a mermaid who they believe is their mother Nerissa and Weilan states her new opinion that the dragon is Nerissa under a spell similar to what took place in the Jao Long Dragon legend. Ondina and Weilan then make a third attempt to get the bracelet. After they find Riki in the moon pool, she tells them the story of how she became a mermaid with two friends and eventually separated from them. Ondina shows her compassion and empathy and is able to convince Riki to let them have the bracelet by convincing her that she is a "Mako Mermaid" just like they are.
As the group prepares to face the dragon for the last time, they look on in horror as a magical storm conjured by the mermaid council begins to destroy the dragon. While Zac and Mimmi face the dragon and attempt to free Nerissa with the bracelet, Ondina and Weilan race to the moon pool beg the council to stop what their doing and give Zac and Mimmi a chance to save their mother. Their pleas prove futile when Veridia informs them that the magic they unleashed is now beyond their control. When both Mimmi and the dragon suddenly disappear, Ondina is completely distraught over the apparent loss of her best friend and starts to break down as she and and an equally devastated Zac desperately cry out to her. However, she is greatly relieved when Mimmi emerges from the water along with a newly freed Nerissa. At Rita's grotto, She and Weilan then look on in great joy as Mimmi and Zac finally reunite with their mother. Later, when Weilan contemplates returning to China to get the eastern pod started again, Ondina offers to join her. Although Mimmi is surprised to hear that she plans to leave Mako, Ondina reassures her that she is only leaving temporarily, wanting to give her and Zac the time they need to bond with Nerissa. As she also contemplates learning to cook Chinese food, the others proceed to poke fun at her for her terrible cooking ability.
Ondina is introduced as a talented, confident mermaid who is determined to save Mako Island and is prejudicial towards mermen and land people. She can be reckless and wilful, and has a problem listening to anyone who doesn’t agree with her. She can also be impatient and headstrong and there are times where even her best friend Mimmi has little patience for her. However, she will usually swallow her pride and admit she is wrong about something when necessary though she rarely apologizes for her occasional mistreatment towards others as pointed out by Mimmi. Although stubborn and single-minded, Ondina is also fiercely loyal to her friends, especially Mimmi. Ondina has a soft side, as seen in her relationship with the merman Erik, which forces her to rethink her prejudices against his kind.
In season 3, Ondina's personality largely remains the same but she no longer harbors any hatred for land people and noticeably more warm to her friends on land. She has also learned to be more compassionate then she previously did. However, she can still be irritable and has trouble warming to new people. Upon meeting Weilan, Ondina harbors an immediate disliking towards her and treats her poorly similar to how she initially treated Zac because of their similar personalities and blaming her for bringing the dragon and later causing Evie to lose her tail. She is later forced to rethink the situation when she causes herself to be stuck in mermaid form in an effort to get Evie's tail back, putting herself in a similar situation as Weilan where she unintentionally caused a problem when trying to help. In "Wishful Thinking" when Ondina and the other mermaids are accidentally sent back in time 5 years into the past by a magic wishing shell and meet a younger Zac, Ondina is hostile and bullying towards him reverting to her previous poor treatment towards him when they first met in the present being impatient to get home and viewing him as an obstacle, much to Mimmi's disapproval. This would prove to be a bad move on her part when they learn from Rita that Zac is the only one who can control the shell that can send them home. When Ondina nervously asks what should happen if Zac hypothetically did not want to help them, Rita reveals they would be stuck in the past forever and Ondina would then realize quickly that young Zac would never want to help her and the others because of the way she treated him. In both of these instances, Ondina takes full responsibility for her poor treatment towards her friends and sincerely apologizes unlike when she rarely would in season 2.
Despite her social flaws and tendency to give a bad first impression, Ondina is a kindhearted mermaid who would go to great lengths to help her pod and friends when they need it. That being said, she's not always the best at expressing gratitude when her friends help her as Zac and Weilan have pointed out (the latter making Ondina repeat herself when she does say thank you to her). She does however have a good sense of humor and is usually fun to be around once she gets to know someone and vice versa. Since coming onto land, she has displayed enjoyment in cooking although she is shown to be a terrible at it which is a running gag throughout her time in the series.
Ondina possesses the ability to transform into a mermaid ten seconds after physical contact of water. She turns back into a human several minutes after climbing out of the the water and once her body is completely dry, she turns back into a human along with the clothes she was wearing before she transformed. As a mermaid, Ondina has the ability to swim at a super speed of 600km/h, hold their breath for more than fifteen minutes, and possibly dive at a depth lower than any ordinary human being.
In "The Seventh Cycle", she sent the Fifty Moons Potion to Zac. In "Stormy Seas", she uses her powers to try to move the boat to her when Ondina and her were having on a contest of mermaid powers. She also did the "bubble trick" during the contest.
In "Stormy Seas", she and Sirena have a contest of mermaid powers and she whipped a dangerous storm over the ocean.
In "Stormy Seas", she and Sirena have a contest of mermaid powers and conjures powerful winds.
In "The Truth About Evie", when she sneezed, she produced a patch of snow.
In "Recipe For Success", she uses this power in attempt to cook underwater marshmallows.
In "Stormy Seas", she uses this power to manipulate the ship on the water with heating the water up and move with it.
- Ondina: "Stop being annoying and I won't."
- Ondina: "The tail was a real giveaway."
- Ondina: "I wasn't very nice to you before. I'm sorry."
- Ondina: " Your just like us. A mako mermaid."
- Her name "Ondina" refers to the Ondines/Undines, a nature spirit that lives in lakes, rivers and seas. Are water elementals. It's sort of mermaid or Tágide, a genius of love, a figure of poetic imagination. The undines also represent the water element in the occult, one of the five elements of the pentagram. Ondina derives from the Portuguese and Spanish word "Onda" that means sea waves.
- As a hatchling, Ondina used to hide under a clam when the full moon came to pass.
- According to Mimmi, Ondina was carried away by a water current when she was a hatchling.
- Like Rikki, Ondina falls for the "bad boy", in this case, is Erik, and both of them also had disagreements with their friends involving their boyfriends, and both of them broke up with their respective boyfriends at the end of the season. Unlike Rikki, however, remained friends with Zane. As Ondina did not remain friends with Erik and he was forced to leave the Gold Coast in shame.
- In the behind the scenes feature, it is revealed that Erik and Ondina's relationship is genuine.
- She isn't very good at cooking or making potions.
- Ondina decided to go to China with Weilan and help her restore the eastern pod.