Umi is the young chosen Sage of the goddess Ondine who is engaged to be wed to Marcella DeLuna, the high priestess of Sinalune, in a symbolic marriage representative of their respective goddess's divine marriage to one another.

It's unclear where Umi's origins lie, as she was found as an infant among a shipwreck that had washed ashore after a terrible storm. The coral-like horns on her head and her unusual pale blue hair color have led to speculation that she may in fact be a half-Syrin, something that has only ever been speculated to be possible.

Umi is the first to greet Aquaries and his party when they arrive on the Sapphire Isle, welcoming them into the Temple of the Goddesses and assisting in their search for Rosalie, Kazimir, and the New Harmonians with them.