Misao Oogami

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.