Chloe From Unknown Seas
The other kids don't know it, but Chloe is a sea ghost, albeit a pretty nice-looking one. She's a turncoat, a double agent, adopting the guise of a gorgeous girl to infiltrate the gang and find out which one of them is the Sea Born.
Because she is a ghost and because she died in the 1950s, she has an otherworldly quality. There is something strange and slightly unsettling about her that you just can't put your finger on. She is very naive and is sometimes overwhelmed by a world of mobile phones, internet and ipods. As a result, she often gets things wrong.
Over the course of the series, as she spends time with the kids, she starts to really like them and comes to question her priorities and devotion to the 'Freedom for Sea Ghosts' cause. This leaves her torn and wondering which side she'd be happier on.
A really evil ghost - Ragnar - is Chloe's 'master' and he is increasingly frustrated with her inability to discover the identity of the island's Sea Born hero. She is too afraid to admit to him that she's not enjoying her mission and feels torn. She knows he will make her 'life' miserable if she betrays him and this is what keeps her serving him.
But by the end of the series, Chloe has a major choice to make that will affect the fate of the gang and the island itself.