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This article is about Cleo Sertori from H2O: Just Add Water. You may be looking for Cleo Sertori from H2O: Mermaid Adventures.

Cleo Sertori is one of the main characters who becomes a mermaid in the Australian television series, H2O: Just Add Water. Along with Emma and Rikki, Cleo receives her powers after her trip to Mako Island.


Cleo Sertori was born in April 19th, 1990[1] to Donald and Bev Sertori. In her childhood, her parents had another daughter named Kim, and Cleo met and quickly became friends with Lewis McCartney when she was five years-old. She also met Emma Gilbert, and becomes good friends with her. Later, she began attending South Coast High School. Cleo loved her birthday parties as a child, but as she grew up, it became too childish until her father knew his little daughter was a bit older.


Cleo Sertori grew up in a family that consisted of herself, her father Donald Sertori, her mother Bev Sertori, and her younger sister Kim. Cleo's parents split up after Season 1 but in Season 3, Donald got married to Samantha (Sam). Don is a successful fisherman and owns three fishing boats. He can employ others into his service. While Don works a lot at the harbor, Cleo's mother stays at home as a housewife. Her stepmother works as a geologist and ship-inspector.

When she was a little older, she met Emma and the two of them became good friends. While her birthday parties seemed fun in the first place, Cleo started hating her birthday when she got much older, because her father and sister organized childish parties. Cleo knows Emma very well, and she also knows that Emma is always prepared.

Cleo's family is not as rich as Emma's family, but Cleo’s father's fishing job enables them to have enough money to live in a house nearby the canal. It's revealed in "Moon Spell" that Cleo is an lactose intolerant. Unlike Emma, Cleo doesn't get along with her younger sister Kim. They argue very frequently, which oftentimes requires their parents’ intervention to reach a medium. Cleo's parents got divorced later in season 1, and in season 3 her father develops a relationship and eventually marries a woman named Samantha Sertori.

It's also revealed that Cleo also has a cousin named Angela, who is even more nasty and manipulating than Kim. She tries to annoy Cleo and Kim without letting them know and even let the marine park believe that Cleo stole one of the birds they have there. This almost causes Cleo to be fired, but Angela gets in trouble, and must promise Cleo to never do stupid things to annoy people anymore. Cleo forces her to do free work at the Marine park by cleaning up the birds’ home at the Marine park.

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

Cleo and Emma were best friends before they met Rikki Chadwick. they both understood each other and both their families get along well. Because Emma's family does associate with Zane's family, Unlike Emma, Cleo takes more distance 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 an small eye on Byron, but she later gave up on him for Emma because she also had an eye on him.

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

The Marine Park

In "Party Girls", Cleo got a job as a dolphin feeder at the marine park and, at the same time, became an ice cream girl. She 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 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.

After the Events of Season 3

In the spin off series, Mako: Island of Secrets, Cleo is indirectly mentioned by Rikki, who appears in the show. Rikki reveals that her friends eventually moved from the Gold Coast after graduating from high school[2].


In contrast to Emma and Rikki, Cleo is often awkward, insecure, emotional and a little shy. 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. Also in Season 2, while working in the Marine Park, the dolphin trainer who is Cleo's boss, helps her train the dolphins. In the beginning of Season 3, she has her own show for the dolphins including Ronnie.

Cleo enjoys being a mermaid, but she is very protective and extremely cautious around water.

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.

Cleo is the most naive and tends to play with her fingers when she gets nervous. However, she is also one of the most encouraging characters. She puts a lot of her faith and support into people even in situations that would appear to be hopeless. Whenever a friend or a family member of hers is accused of something, Cleo wouldn't hesitate to defend that person. Unfortunately in certain situations, 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.

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, even to the point where she tries to make sure everyone else does their work too.

In addition, season three shows how Cleo’s character has truly transformed. She becomes more confident, encouraging, independent, and optimistic. Near the series’ end - episodes seventeen and onwards - when Rikki develops a connection to the water tentacle (unbeknownst to Cleo and Bella), Rikki verbally hurts Cleo by accusing her of being irresponsible with their mermaid identity in front of Ryan. While Cleo was truly protecting their secret, Cleo maintains a confident composure and voices her concerns to Rikki. When she didn’t want to hear anymore, Cleo would walk away from their conversations in an effort to leave both sides to cool off. In the end, the girls would make up and their mermaid secret would remain protected.

Physical appearance

Cleo Sertori was a teenage girl by the events of Season 1, and was by this time she also became a mermaid.

She has wavy-curly brown hair that stops mid-back and hazel-green eyes. 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 styles such as being simply flat-ironed, curled, braided in the front 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 tall standing at 5'7''. Unlike Rikki and Emma, Cleo used to be the "girly girl" of the former mermaid trio.

As a mermaid, her hair is let down and she has an orange-scaled tail with an orange-scaled top.

When she worked 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 was training dolphins for shows.

She wears a lot of colorful clothes based on her girly (at first) personality, which matured more and more as the show went on. Her based colors are: purple, lavender, etc. though she seems to wear different shades of blue throughout season 3, and even tries bolder colors like red and black. Cleo also gets a pair a glasses towards the end of the third series.

She also wears a locket identical to Emma and Rikki's, only with a blue stone, to represent the ability to control water. However, before Emma's departure in Season 3, they switched their lockets.

Towards the end of season 3 she wears a crystal necklace that was given to her by Will. However, it is uknown if she still wears it or got her original locket back by 2016, when Rikki Chadwick, one of her best-friends, came back to Mako Island.

Becoming a Mermaid

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 minutes 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 hollow mountain. Emma realizes that the Moon Pool is connected to the ocean and may be their only way out, and 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.

Mermaid Powers

Cleo possesses the ability to transform into a mermaid ten seconds after physical contact with water. She wasn't able to transform into a mermaid until she made contact with the water of the Moon Pool at Mako Island about 12 hours later. She turns back into a human several minutes after climbing out of the the water. She turns back into a human along with the clothes she was wearing before she transformed when she's dry. As a mermaid, Cleo has the ability to swim at a super speed, and dive at depths no human being is capable of, completely protected from its cold and water pressure. She is also capable of holding her breath for at least fifteen minutes. When Isabella Hartley arrived at the Gold Coast just after Emma's departure, Cleo and Rikki realized how beautiful her crystal necklace was.

The new mermaid trio then replaced the original mermaids' lockets with crystal necklaces that Will Benjamin handcrafted for both Cleo and Rikki. These necklaces hold the power of the full moon and granted Cleo and the girls the ability to mimic the effects of the full moon which put them under the Moon spell when in contact with each other. It is unknown if Cleo still wears hers or not.

Due to entering the Moon Pool with Emma and Rikki, Cleo was only gifted with the power of Hydrokinesis, and didn't receive all merpeople powers like a person who enters the Moon Pool alone. In Season 2, after a powerful planetary alignment, Cleo was gifted with a second power called Aerokinesis, and later, she was able to create a storm with Emma and Rikki. Under the Moon Spell, Cleo was temporarily turned into a Siren, thus gifted with the ability of singing enchanted songs, however, it only lasted when the Full Moon was up, and her ability was gone when the Full Moon set.


Before her transformation, Cleo had all the typical weaknesses of a human. When she transformed, she now is susceptible to all the weaknesses of a mermaid. Due to being once a human and going through metamorphosis in a Moon Pool, Cleo is susceptible to all the Full Moon's special events on a Moon Pool, like the temporary twelve-hours Lunar Eclipse or the dangerous power-stripping Moon of Fifty Years.


Lewis McCartney

Cleo and Lewis sharing a romantic kiss during graduation

See the main article: Cleo-Lewis Relationship

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 very 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. She also briefly became bitter to Lewis as well, by treating him as if he was betraying her by hanging out with Charlotte, acting as if they had never broken up and that she was being cheated on. This is largely due to the fact that Cleo always regretted breaking up with Lewis. 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. Following this, the rift between Lewis and Cleo only grows for a moment. Gradually however, Cleo begins to accept that she has no rational reason to be angry with Lewis for being with someone else since she broke up with him. The two of them eventually return to being extremely close friends and Cleo accepts his relationship with Charlotte.

After Charlotte became a mermaid, her true colors began to surface and she proved to be quite venomous. She isolated Cleo from Lewis and broke her spirit by making her believe that she's pathetic and that Lewis not seeing her anymore was his own choice. To add insult to injury, Charlotte also stole Cleo's locket. Charlotte also deceived Lewis into thinking that Cleo gave her the locket by choice, causing Cleo to believe Lewis truly doesn't care for her anymore when she sees Lewis put it on her and then kiss her. Cleo then leaves Lewis a message expressing her heartbreak and disappointment in him for abandoning her before running away from home.

When Lewis found out the truth, he broke up with Charlotte and went to save Cleo from the sharks. He then apologizes for hurting her and 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 Tate

Cleo And Ryan

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.

Behind the scenes

She was portrayed by actress Phoebe Tonkin.

See also


Cleo is a Greek name and the meaning is "Glory". It is the alternative form of Clio.

In Greek mythology, Clio refers to 3 figures: A Muse and goddess of history and poetry; a Nereid (sea nymphs); and an Oceanid (another sea nymph and each was an individual patroness of particular spring, river, sea, lake, pond, pasture, flower or cloud).


  • Cleo, Rikki, Lewis, Zane, Nate, Don, and Kim are the only people to appear in every season.
  • Cleo is lactose intolerant.
  • Before becoming a mermaid, Cleo could not swim, and she was afraid of water in general.
  • In season 1, she had bangs, but did not in mermaid form. In the two seasons after, she showed her temple a lot, and was still bangless in mermaid form.
    • There some scenes in Season 1 where she wears a headband or letting herself show her temple naturally.
    • Also in the season 2 intro, the two clips from Metamorphosis where Cleo does have the bangs is in there. But in the episodes themselves, she is bangless.
  • At the beginning of the episode "Surprise!", a calendar is shown, where Cleo's birthday is revealed as April 19th, 1990[1].
  • Cleo's dad is the only parent known to remarry.
  • It is ironic that Cleo, both before and after she became a mermaid, was always close to water even though she had to be careful. For example, helping Emma train or working at the Marine Park.
  • Cleo was promoted in her job as a dolphin trainer in Season 3.
  • Emma gave her locket to Cleo before she left for her trip around the world.
  • Cleo can be seen wearing Emma's locket in Season 3.
  • Cleo also gave her locket to Emma.
  • Cleo is very good at science.
  • Cleo was the first to discover her power over water.
  • She was the last girl seen in mermaid form.
  • Of all the main girls in the series, including Emma's replacement, Cleo has cried the most.
  • Cleo was introduced as shy but became more confident in Season 2 and 3.
  • Cleo is the only girl who was never taken by the Water Tentacle. Though it tried to attack her in her bathroom, it was not able to grab her or take her to the Moon Pool.
  • Cleo was the only one of the girls to graduate with a biology Major or a Major in general.
  • When Cleo first swam in the ocean in "Catch of the Day", she panicked. She held her breath and tried to swim up to the surface, but her tail was weighing her down until she realized she now could hold her breath longer. She then learned how to swim with a tail.
  • Her father's nickname for her is Chooks.
  • Her western zodiac sign is Aries and her Chinese zodiac sign is a Horse.


Main article: Cleo Sertori (JAW)/Gallery


  1. 1.0 1.1 H2O: Just Add Water: Season 1: Episode 14: Surprise!
  2. Mako: Island of Secrets: Season 3: Episode 16: Homecoming