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 by Sou Fueki during the Sabbath. As a result, as by-product of Fueki's attempt to revive his daughter Koyomi, 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, knowing Fueki through his Wiseman alias and seeing his word as law, Medusa works together with the more defiant Phoenix to create more Phantoms from Gates.
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 who takes the girl under his wing.
Eventually, forced to work under Sora, Medusa finds herself facing Mayu after gaining the ability to become Kamen Rider Mage. It was only while going after Yamamoto, mortally wounded when Mayu used the Holy Ring on her, Medusa attempted to kill her when she was literally stabbed from behind by Wiseman. Feeling betrayed, she asked him why, Medusa is horrified to find out that Wiseman is also the White Wizard and dies with the knowledge that she meant nothing more to him than a pawn whose services are no longer needed.
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. Medusa also loves Wiseman, becoming hysterical in her final moments.
- Medusa bares some similarities to Jihl Nabaat as a right hand woman with a sadistic streak who is eventually killed once no longer needed.