Cleo Setori, Mermaid of WavesCleo Sertori (Phoebe Tonkin) is the brunette of the group and a bit awkward, but eventually becomes more outgoing. Her powers are manipulating the shape of water and she later gains the Razor Wind power too.
Emma Gilbert, Mermaid of IceEmma Gilbert (Claire Holt), the "leader" of the group, who is a rather serious an responsible girl. She has powers to freeze water and manipulate ice, snow, and clouds. Her actress is noticeably missing from the third series; she left to film a straight-to-video prequel to The Messengers.
Rikki Chadwick, Mermaid of FireRikki Chadwick (Cariba Heine) is the "Wild Child" of the group, who in appearance and attitude is similar to Sam from iCarly (although Rikki notably pre-dated her). She is the new girl in town but not a Na´ve Newcomer, because she is rebellious and confident. Her powers initially involve manipulating heat in water (i.e. boiling it), but eventually extend to control over fire and lightning.
Bella HartleyA mermaid since the age of nine, Bella moves to Mako Island from Ireland in Season 3. Her powers are turning water into gelatine, making water harden into crystal, and to densely compact particles to make substances explode.