Madama Styx

Madama Styx, also known as the Decider of Ends is an Demon charged to lure the souls of the sinners to Inferno. She is also the Demon who made a pact with Jeanne and is the source of most of her powers.

Bayonetta
Like Madama Butterfly, only her limbs are visible in the first game and her role in the story is only to assist Jeanne during her fight against Bayonetta and remplace Madama Butterfly when the player choose to play as Jeanne.

Bayonetta 2
Madama Styx's body can now be fully summoned when you play as Jeanne. In regular story mode, only her limbs are visible but if Jeanne is choosen as the playable character, Madama Styx will replace Madama Butterfly in the cutscenes.

Book of Infernal Demons
"Queen of the River Styx, the boundary between humanity and Hades. The moths born from the magical power that flows from her body are charged with luring the souls of sinners into Inferno. By trading their soul to Madama Styx, one is able to obtain great power and knowledgeable counsel."