She is also girlfriend of Lewis McCartney.
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. He can employ others into his service. While Don works a lot at the harbor, Cleo's mother stays at home as a housewife. Cleo claims that she knew Lewis McCartney since they were five years old, and she never became angry or frustrated on him ever since she met him. 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 really good prepared.
Cleo's family is not as rich as Emma's family, but Cleo her 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 always are fighting or acting uggly towards each other and cause their parents to get crazy. 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 Roberts.
It's also revealed that Cleo also has a cousin named Angela, who is even more nasty and manipulating than Kim. She tries 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 become fired, but Angela gets in trouble, and must promise Cleo to never ever do stupid things anymore to annoy people, and 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 an 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 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.
After the Events of Season 3
In the series of Mako: Island of Secrets, Cleo doesn't appear, but is remembered by Rikki, who appears in the show. Rikki reveals that Cleo eventually moved from the Gold Coast after succeeding high school. It is unknown what happened to her, but it is possible that after finding a new passion in science, she left for America to start a career there and also to be with Lewis.
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. 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.
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-curly 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 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 color is pink, and her based colors: purple, lavender, etc. though she seems to wear different shades of blue throughout season 3. 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 water's controlling ability. But in the beginning of Season 3 she wears a locket with a white stone which was known to be Emma's locket in Season 1 and 2. Presumably, she and Emma switched their lockets before Emma left to travel the world with her family as a symbol of their friendship. Towards the end of season 3 she wears a crystal necklace that she got from Will.
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 dormant volcano. 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.
Cleo possesses the ability to transform into a mermaid ten seconds after touching water. She wasn't able to transform into a mermaid until she made contact with the water of the Moon Pool at Mako Island 12 hours after the contact. 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 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 control and manipulate 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, making an water ball or spray it in a form of a water jet blast. To shape the water, she makes a water ball that is floating in the air, and after that, she can shape it into anything like a miniature mermaid figure. Her powers allow her to make a tube from water, and creating more water than the original amount of water by manypulating the water monucules, and let the tube grow as big and high as she wants.
In the second season in the episode "Stormy Weather", a special full moon upgrades the mermaids ther powers. Now, Cleo can also controll the wind. When combined with Emma and Rikki's powers, she can produce hurricane-like weather. She uses this along with Emma and Rikki to stop Charlotte and let her fall into the moon pool during a full moon so she loses her powers.
In season three, Cleo is the only mermaid who doesn't display her secondary power, but because Rikki is able to generate heat on solid objects, it is possible that she also 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 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. With this power, Cleo can control and manipulate the air and wind in her favor.