Nagisa Sagan

She is an easily angered woman who conducts experiments and creates humans who cannot feel physical pain. Although Nagisa is very young in appearance, it is later revealed that Nagisa is over 30 years old. Her background, and affiliation are unknown. She had a sister, but what has become of her has not yet been confirmed. She created Youji after the image of her younger sister, perhaps in remembrance or to ease the pain in her heart, but exact reasons are still unknown. Her over-the-top behavior and strange lifestyle leaves her alone and alienated. She befriends Nana, and shamefully confesses that she is her only friend.

Her relationship with Ritsu is an intriguing one. She puts up a front of hating him deeply, and wishing for nothing more than to see him fail. However, there are small moments that reveal possibly more intimate feelings, as she blushes deeply whenever he is near her or touches her, though she steadfastly denies caring for him. Starting in volume 3 and finishing in volume 4, she makes a bet with Ritsu that her older Zero pair, Yamato and Koya, can beat Ritsuka's Soubi. Ultimately, Yamato and Koya are defeated, and Nagisa's sibling rivalry-like attitude towards Ritsu continues.

(Source: Wikipedia)