Lewis McCartney is one of the main characters in H2O: Just Add Water. Emma Gilbert is one of the main characters who became a mermaid in the. Phoebe Jane Elizabeth Tonkin (born 12 July ) is an Australian actress. Cleo Sertori and Lewis McCartney's relationship is one of the relationships in the H2O: Just Add Water series. Cleo wants to tell Lewis that she's a mermaid and wants to show him her powers with water. Emma and Rikki refuse to let her to tell that to him or anyone else about her. Kim Sertori is Cleo's annoying younger sister who is always snooping around. H2O: Just Add Water, also known as H2O, is an Australian dramatic fantasy television Rikki Chadwick, Emma Gilbert, and Cleo Sertori are three teenage. 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.