Cleo Sertori


Portrayed By:
15-16 (Season 1)
16-17 (Season 2)
17-18 (Season 3) Birthday: March 6th
First Appearance:
Last Appearance:
Don Sertori (father)
Bev Sertori (mother)
Samantha Roberts-Sertori (step-mother)
Kim Sertori (younger sister)
Angela (cousin)
Best Friends:
Love interest:
Lewis McCartney (boyfriend)

Cleo Sertori is one of the main characters who became a mermaid in the Australian television series, "H2O: Just Add Water". Along with Emma and Rikki, Cleo discovered her powers after her trip to Mako Island. She was portrayed by actress Phoebe Tonkin. Her birthday is March 6th and she is a Pisces.


Cleo Sertori grew up in a family that consisted of herself, her father Donald Sertori, her mother Bev Sertori, and her younger sister Kim. Don is a successful fisherman and owns three fishing boats and can employ others into his service. While Don tends to work a lot at the harbor, Cleo's mother stays home at as a housewife. Cleo claimed that she had known Lewis McCartney since they were five years old and never once had she seen him get angry or frustrated. Later on, she met Emma Gilbert and the two of them became good friends. While her birthday parties seemed fun at first, Cleo came to despise them as she got older, because her father and sister insisted on giving her childish parties. The two knew each other well enough where Cleo knows that Emma is always prepared and Emma knows that she is tone deaf. In one episode, Rikki Chadwick describes her and Emma as having "The Perfect Families" but Cleo quickly protests.

Although not as wealthy as Emma's family, Cleo's father's fishing job enables them to have enough money to live in a house by the canal. It is revealed in "Moon Spell" that she is lactose intolerant. Unlike Emma, Cleo doesn't get along with her younger sibling. At the most, they would both argue and get on each others' nerves as much as possible to the point where it would drive their parents insane. Cleo's parents got divorced late in season 1, and in season 3 her father develops a relationship with and eventually marries a woman named Samantha Roberts.

Aside from her family, it is revealed that Cleo also has a cousin, Angela, who is even more deceitful and manipulating than Kim when she tried to trick them into accusing each other while she went body-boarding herself.

Her friendship with Lewis remained platonic, although Lewis had developed a crush on her later on in season one. Cleo doesn't respond to his crush until the end of season one, but she continues to display a caring attitude towards him and would encourage him many times whenever he was feeling down.

Because Cleo and Emma were best friends before they met Rikki Chadwick, they both understand each other more and both of their families get along well. Because Emma's family does associate with Zane's, Cleo and Lewis come to be familiar with him as well.Unlike Emma, Cleo is more wary of Zane, especially due of the fact that he kept pulling pranks on her in season 1.

In the first few episodes of series one, Cleo seems to have a small crush on Byron, but she later gave up on him for Emma's sake.

Before she became a mermaid, Cleo was afraid of the water and could barely swim. She also tends to rely on Lewis for academic success in the first two seasons. After he leaves to study in America, Cleo took it upon herself to study more and was able to achieve an honorable diploma, and after the ceremony her, Bella and Rikki see Lewis coming down the steps of the school to see Cleo, who is still in love with him.


In contrast to Emma and Rikki, Cleo is often awkward, insecure, and a little shy. She is the most "girly" out of the three and is well intentioned. She is the most empathetic out of all three girls. Cleo loves her fish and enjoys taking care of them extensively. When Pluto, one of her salt water fish, died, Cleo became saddened by his departure. Unlike Rikki, Cleo believes that fish are just like people that live, eat, and play underwater. Another sea mammal that she is mostly empathetic to are the dolphins. At the Sea World Marine Park where she works part-time, Cleo shares a close bond with a dolphin named Ronnie.

Unlike Emma, Cleo tends to lack confidence when it comes to getting things done. Often times she would need the right push and encouragement to be able to perform a task she once thought impossible. Unlike Rikki, Cleo tends to be more responsible and less rebellious, but she does show some display of being carefree from time to time. Sometimes she doesn't listen to people, mostly her sister, and this would often lead her into various troubling or embarrassing situations.

Aside from her girly personality, Cleo is the most naive and tends to play with her fingers when she gets nervous. She would put too much of her faith into people even in situations when it seemed to be hopeless. Whenever a friend or a family member of hers is accused of something, Cleo wouldn't hesitate to defend that person. Despite knowing the worst, Cleo would continue to deny the negative truth being pointed out by her friends. All in all, Cleo would believe in her friends and family right up to the end.

Often times Cleo was the one being stuck in the middle of an argument between Emma and Rikki whenever they fought. Often playing the peacemaker, she tends to find herself torn between her two friends, especially when she insists on not taking one side or the other.

During season three, Cleo begins to focus more on science due to Lewis' departure and her desire to find out where the water tentacle came from. Because of this, Cleo begins to study more and have almost the same personality as Emma, even to the point where she tries to make sure everyone else does their work too.


Cleo has wavy brown hair that stops mid-back and hazel-green eyes. She is quite beautiful despite some of her insecurities. Throughout the series, Cleo would often arrange her hair in various styles ranging from simple buns, braided, loose, or pony-tailed. In season three, though, she tended to wear her hair a bit more maturely, and less messy, and styles such as being simply flat-ironed, curled, or in a pretty pin up, were much more common. Her skin is clear and is an extremely light tan color. She has a slender figure and is quite tall standing at 5'10.

As a mermaid, her hair is let down and she has an orange-scaled tail with a matching orange-scaled top. When she works part time at the marine park, Cleo would wear a light blue polo shirt with a name tag and a navy blue baseball cap. In series three, she wore a darker navy blue and black wet suit whenever she's training dolphins for shows.

She wears a lot of colorful clothes based on her ability on moving water into shapes similar to the colors of the wind. Her favorite colors are purple and pink based on her clothing, though she seems to wear different shades of blue throughout season 3.

In season 1 and 2 she wears her silver locket with the blue gem, but in the beginning of season 3 she wears a silver locket with a white stone which was known to be Emma's locket in season 1 and 2. Presumably, she and Emma switched lockets before Emma left to travel the world with her family. Towards the end of season 3 she wears a blue crystal necklace that she got from Will.

Becoming a MermaidEdit

Cleo becomes a mermaid in first episode "Metamorphosis" of season one. After helping Emma train by timing her laps, she walked home and was tricked into helping Zane Bennett repair his boat. Deciding to pull a prank, he and Nate set her adrift in his spark plug-less boat and into the sea, knowing that she could barely swim. Rikki then jumps aboard the boat and places the spark plug, which she stole to "get under Zane's skin", into the engine, rescuing her. Cleo and Rikki then take the boat for a joyride. Along the way, the girls ran into Emma and invited her to come along. With the carefree Rikki steering the boat out into the sea and miles offshore, the girls soon found out that they are now out of fuel in the middle of the ocean. With little choice, the three girls paddled their way to the closest land they could find: Mako Island.

Once ashore on the deserted island, Cleo and her friends pondered on how they should call for help. Being the most prepared one in the group, Emma brought out her mobile phone but only to find that it couldn’t get a signal. Emma suggested that they should head inland towards a higher altitude in hopes of getting a signal for a call. As they ventured in deeper into the jungle-infested island, the three girls soon came across a clearing where a river stream turns into a waterfall over several boulders. While Emma and Rikki were able to jump over to the small waterfall, Cleo became hesitant and without a warning, fell into a hole. The other two followed her and continued to pursue a way out of the underground cave.

Several hours later, the three of them ended up at the end of the tunnel and found the moon cave with a pool at the bottom of the island’s dormant volcano. Emma realizes that the Moon Pool is connected to the ocean and may be their only way out, dove in to find out. Confirming her find, she and Rikki convinced the hesitant Cleo to escape with them. When all three of them entered the pool, the full moon became aligned with the moon pool, causing it to bubble and glow a mystic blue glow and emitting small orbs of magical light particles into the air. Oblivious to what was happening to them, the girls ventured out into open sea where they were rescued by the local water-police.

Upon the next morning and twelve hours after their ordeal, Cleo decided to take a morning bath. To her surprise, she transformed into a mermaid with a large tail hanging at the edge of her bath tub only ten seconds after touching water. Cleo later confirms that she wasn't the only one who became a mythical sea creature as Emma and Rikki both became mermaids as well.

Cleo is the last person to embrace her new life as a mermaid while the other two are excited, despite the obvious downside. However, she later got over her fear and hesitation and tried her best to live a normal life and was able to grow to love her new abilities.

The Marine ParkEdit

In Party Girls, Cleo got herself a job as a dolphin feeder at the marine park, at the same time, she becomes an ice cream girl and would occasionally help the other staff members feed the sharks and the other tank animals. At the marine park, Cleo knew that she has to stay away from water-filled rides. At times, she would even swim with the dolphins behind the other staff members' back. It was there that Cleo met Ronnie the dolphin and the two began to become friends and share a deep friendly and playful bond (although at first, she thought that Ronnie had a crush on her after seeing her as a mermaid). In the third series she was promoted as a dolphin trainer, but even that took her a while to master fully without Will's help.

Mermaid PowersEdit

Cleo possesses the ability to transform into a mermaid ten seconds after touching water. She wasn't about to transform into a mermaid until an estimate of twelve hours after jumping into the Moon Pool at Mako Island. 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, Cleo has the ability to swim at super speed of 600km/h, hold her breath for more than fifteen minutes, and dive at a depth lower than any ordinary human being is capable of.


Cleo is the first of the three mermaids to discover her individual powers over water; the ability to move, shape, and bend water at her will, normally called Hydrokinesis. Her ability ranges in many forms. When moving the water, she can cause it to float in the air or spray it in a form of a water jet blast. To shape them, she manipulated the water into the air and cause them to form from simple water orbs to a miniature mermaid figurine. Her powers also allows her to multiply the water molecules, making the shape of the water to grow more and expand in sizes, despite the small amount in a glass to begin with.

Power EnhancementEdit

In the second season premiere "Control", a special full moon along with a rare planetary alignment greatly strengthens and increases her powers to much more powerful. In Cleo's case, she develops the ability to create and control the wind, most commonly known as "Aerokinesis". She can also control rainstorms and create them over a person or an large area.

When combined with Emma and Rikki's powers, she can produce hurricane-like weather. However, it appears that Cleo's power alone isn't enough to levitate a person or anything heavier- unlike Charlotte Watsford who could levitate all three girls at once, it takes all three to levitate one person.

In season three, Cleo is the only mermaid who doesn't display her secondary power, but seeing how Rikki is able to generate heat on solid objects, it is possible that she still has them. It is most likely that in the episode "The Sorcerer's Apprentice", Cleo couldn't use her aerokinesis to blow her bubble-imprisoned goldfish back towards her, because she was already trying to get a grip on the floating bubble with her hydrokinesis. This implies that it is possible that Cleo and the girls can't use both of their powers at the same time unless when combining their powers with each other.


Lewis McCartneyEdit

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Cleo and Lewis became friends since they were five years old. As they grew up, Lewis developed a crush on her but she was only interested in being friends with him. Later, Cleo admitted that she's falling in love with Lewis and the two started to date at the end of Season 1 after they admitted their feelings for each other. At the beginning of Season 2, after Cleo got her new powers, Lewis becomes clingy and doesn't give her enough space. Cleo then breaks up with him to find the balance. When Lewis begins hanging out with Charlotte, Cleo grew jealous. Charlotte's new friendship with Lewis made her appear closer to him in hobbies and interest, causing Cleo to become more and more weary of Charlotte. When Lewis tried to divert Charlotte's attention away from Cleo on Mako Island, Charlotte took her opportunity to kiss him and making them an official couple. Cleo is heartbroken and accepted the fact that Lewis is moving on away from her, not knowing that Lewis only did it to protect her mermaid secret from Charlotte.

After Charlotte became a mermaid, she isolated Cleo from Lewis and broke her spirit by making her believe that she's pathetic and stole her locket. When Lewis found out the truth, he broke up with Charlotte and went to save Cleo from the sharks. He then admits his true feelings for Cleo and they both got back together. Cleo and Lewis continue to maintain their relationship in Season 3 but when Lewis receives a scholarship from America, Cleo didn't want to let him go. But knowing that she can't be the reason to keep him from achieving his dream, she accepted the choice to let him go. Though they have departed, Cleo and Lewis still remain in contact with each other. At the end of Season 3, Lewis comes back from America to visit Cleo at her graduation. They get together and end up at the juice bar. They kiss and dance to Bella's music.

Ryan TateEdit

Shortly after Lewis' departure, Cleo develops a crush on Ryan Tate, a geologist who works with her step-mother, Samantha. Ryan anonymously gifted Cleo a telescope, what she thought to be from Nate. Kim encourages her to date Ryan, but Cleo decided to remain loyal to Lewis. However, she becomes wary of him when his suspicions on Mako Island goes too far.