Cleo Sertori is one of the three main characters in Nickelodeon’s H2O: Just Add Water. The series takes place on the Gold Coast of Australia. A strange encounter with the magical moonpool on Mako Island turns the girls into mermaids! Now Emma, Cleo, and Rikki must navigate life as a human on land and mermaid in the water. Cleo is the kind-hearted one of the group, though she can be anxious. Her mermaid power is manipulating water. Phoebe Tonkin, known for her later role in The Originals, portrayed this character in all three seasons.