Medusa is a Phantom, a type of monster that emerges from Gates, magically endowed people when they fall into despair.
Medusa was born the day her original self Misa gave into his despair and she emerges, killing Misa in the process. Since then, able to detect Gates, Medusa worked under Wiseman to oversee her kind's progress while directing each to their ideal target.
Medusa is cruel and calculating, not above harming fellow Phantoms if they stray from Wiseman's orders. She also has no tolerance for anyone who gets in the way of her kind's goal.