Ооґамі Місао

Misao is a girl from the Kannagi household. Although a kind person at heart, having saved Kazuma from bullying ten years ago, Misao blames Kazuma for her brother's death and attempts to kill him several times. Her attempts fail, and she becomes a pawn in Michael's plan to take Kazuma down. [Source: Wikipedia] The power Michael gives her lets her suck the life out of anyone to increase her own powers. This also has the effect of warping her powers and personality, causing her to emit a dark, sickening aura. With her new power, Misao even kills almost a hundred people to increase her power and even kills her own father and several members of her family with no hesitation when they attempt to stop her.

After being defeated by Kazuma and realizing that Michael was only using her, Misao is absorbed into a monster that Michael summons using the power that she gathered from all the people she killed. Fortunately, Ayano is able to save her by using cleansing flames (with help from a crontracted Kazuma) to kill the monster and free Misao. Realizing the depth of her mistakes, Misao resolves to become a shrine nun for the rest of her life to make up for all she had done. She does appear to finally forgive Kazuma at that time, saying that she had still not yet paid him back for all he had done for the Kannagis, and that she did promised that he could do anything he wanted to her, blushing as she said so.