Kappa is the Beacon, a gift from the Land God. He corrects people who call him “divine, great, or The Beacon”. He feels that they should know his name, and treat him as an equal, not a God.

The Beacon who doesn’t wanna be a Beacon

Siren is the prince of the Sharks and one of the protagonists of Castle Swimmer. He was supposed to kill Kappa as part of a prophecy but chooses to save him instead. He is currently on a journey to find his Father, and to find a way to break the Sharks' curse. Like Kappa, he is a merman. He has a back fin, and sharp teeth, but he’s very friendly, and polite, like a formal price. However, he has no knowledge of the outside world, and got most of his information from Kappa, and by his travels.