Lewis: "Cleo, everybody is scared of something. But you, you're amazing, you can do anything."
Cleo: "Just because I've got these powers doesn't mean I'm amazing."
Lewis: I didn't mean the powers."
-episode Catch of the DayCleo Sertori is one of the main characters who became a mermaid in the Australian television series, H2O: Just Add Water. Along with Emma Gilbert and Rikki Chadwick, Cleo discovered her powers after her trip to Mako Island. She was portrayed by actress Phoebe Tonkin.
Cleo is kind and compassionate girl who wears her heart on her sleeve. She is the most empathetic of the mermaids and often takes the role of mediator when Emma and Rikki clash. Cleo is often awkward, insecure, sometimes shy. 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.
However, she also possesses great strength and courage. She faced her fear of water to save turtles from being caught and drowned, she took a job at the Marine Park and refused to let water make her live in fear. She even encouraged Emma to not let her fear of discovery keep her from living. While responsible, Cleo does have a playful side and once she overcame her fear, she grew to love her mermaid life. Despite her gentle nature, she does have a temper. She would often splash Kim whenever the latter annoyed her and even used her powers to sabotage what she thought was a dinner with a future stepmother and sister.
Even before she became a mermaid and overcame her fear of water, Cleo showed great compassion for animals, particularly aquatic ones. She loves caring for her own fish and was heartbroken when one of her fish, Pluto, died. Cleo also shares a close bond with Ronnie, a dolphin at the Sea World Marine Park.
As the series progresses, Cleo matures and becomes more confidant in her self and more driven. In Season 3 her interests grow and develop. She is promoted to dolphin trainer and even gets her own show. She also develops an interest in and love of science, not unlike Lewis. She also became the "mother hen" of the group, probably from having to help her father and sister after her mother left, often urging the others to study for their exams and worrying when they didn't. In contrast to Season 2, Cleo was more open and accepting of a new mother, only wanting her father to be happy. She also stopped splashing Kim. As she matured, Cleo's ability to empathize and show compassion also grew.
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, 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''.
As a mermaid, she gains a long fish-like tail covered in shimmering scales and a top made from the same scales and fins. From Season 1 to 2 Cleo's tail is coppery orange but becomes a brighter, more golden color in Season 3. Like the rest of the girls, her skin is glowing and ethereal when in mermaid form.