One of the Wizards of DEM (Deus Ex Machina), an organization which provides realizer units to other wizards throughout the world to fend off spirits. When DEM was transferred to Tenguu City's AST, she was posted on that spot to deal with the Spirit called Nightmare. Mana doesn't recall much of her past and it is said that she's one of the best wizards existing.
In Volume 3 it is revealed that Mana is Shidou's blood-related sister; though she can't recall much of her past 5 years ago. She has a photo of her and Shidou that reveals that Shidou is her brother. Even though Mana is one of the best wizards existing, DEM has been experimenting on her body via tests and drugs; making her capable of using other tools meant to kill Spirits. It takes a side effect on her body, but she herself doesn't know it.
In Volume 4 she was taken into Ratatosk for custody; in Volume 5 she joins Ratatosk.