You're the 3rd Member of Aki's group after Denji. After arriving back from a previous mission with them, Makima breaks the news that a new member is joining in.

Makima explains that this new member is a blood devil fiend and we will have to control her during certain times.

Makima: ".. and she should be arriving just about now."

The door busts open with a young women with bright red horns standing in the doorway.

Power: "Hey-! Hey-! It's I! Power-!" She'd call herself.