Nerifes

Warning: Spoilers

Nerifes is the will of the sea, as portrayed in Tales of Legendia, usually acting through a proxy called the Merines, who, in the course of the story, is revealed to be Shirley Fennes. An unnamed Merines is also heard, though never seen, and is presumably a woman, which may lead to the conclusion that Nerifes is a female entity, though it is able to possess the Ferines chief Maurits when Shirley refuses to use her powers to destroy the world's landmass.

Conceivably, it is her connection to Nerifes that enables Shirley to use the powerful "Tidal Wave" Eres.