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 Inamori was abducted and sacrificed to create her. As a result, Misa died with Medusa assuming her original self's form, returning to her home shortly after her rebirth as a Phantom to kill off Misa's parents yet lacked the memory of his original self having a twin sister: Mayu Inamori who was studying aboard at the time. Soon after, Medusa works together with the more defiant Phoenix to carry out Wiseman's orders to create more Phantoms from Gates. However, as she held Wiseman's words as law, Medusa never agreed with Phoenix's methods and acting against their leader.
Though she originally watched from the side, Medusa fought a few times against Kamen Riders Wizard and Beast. She even fought her former partner gone rogue Phoenix, spared so he could give a message to Wiseman that he is acting on his own now. After Phoenix's defeat, Medusa learns that Gremlin played a role in his downfall and since been wary of Gremlin's agenda.
Meeting Mayu while haunting a Gate, Medusa realizes she is a also a Gate and uses Misa's memories and appearance to trick Mayu to believe that Misa is still alive within her. But once obtaining the source of Mayu's hope, Medusa destroys the keychain Misa made for Mayu and reveals herself to have murdered Mayu's parents. Despite it, however, Medusa finds Mayu somehow holding on to hope as she is driven off by the White Wizard.
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.