Phoebe Tonkin

Emma, Cleo and Rikki are sixteen anno old girls growing up on the sun-drenched beaches of the oro Coast. Emma is confident, brilliant, responsible, and athletic, Cleo is slightly insecure, sweet, and awkward while Rikki is the aloof and rebellious new girl.

The girls find themselves stranded at sea one day, floating towards the mysterious Mako Island. The three girls explore the eerie jungle-covered island before becoming trapped in an ancient cavern under a volcano. Discovering an underwater channel, they decide to swim to safety. As they enter the pool, the light from a full moon illuminates...

I was rewatching H2O season 1 when I realized something: when te think about, each original character has had their own season that sort of resolved around them. Let's start with the beginning:
Season 1: Emma
You may o may not have noticed but the one who always gets seen da Zane is Emma. She is the one who saved Zane, she is the mermaid that Zane saw twice (during his saving and with her red hair) and in the finale she told dr. Denman to go home while Cleo and Rikki just walked away.
Season 2: Cleo
This is not rocket science. The main villain of this season existed da the cause of the Amore triangle...