Charlotte, as a mermaid
, introduced in Season 2, is the main antagonist of H2O: Just Add Water. At first really nice, her jealousy for Cleo (having a huge crush on Lewis) and eventually, becoming a mermaid and letting her powers take over her create her dark side.
She has all three
powers, and possibly Bella's powers, which weren't known at the time. Also has complete Atmokinesis.
Her grandmother was one of the three original mermaids. She first appeared in Fire and Ice and last appeared in The Finale.