She is a butterfly fairy embodying the life cycle of a butterfly. Her abilities let her scatter scales. She is normally very gentle, but the incident has empowered her to be a bit more aggressive than usual.
In Cirno's Extra route, Okina speculates that Eternity may not be a mere fairy, but is in fact a Tokoyo Kami (or God of the Everlasting World) and one of her enemies.
