A 10th grade student. Originally about to be executed by the devils after her sister, Kuroka, kills her own master. The Satan, Lucifer, protected her, and left her under his sister's, Rias' care - becoming her servant. She refused to use the power of youjutsu because she didn't want to become like her sister. She overcame her power along with Akeno after Ise made her overcome her own powers. She doesn't show any emotion at all, and says harsh things and hits Ise when he is doing or thinking something perverted. She shows the most concern for her comrades when they are in trouble.
SpoilerAt the end of Volume 5 she has warmed up significantly to Issei and even begins sitting on his lap and sneaking into his bed. After this, she gradually develops romantic feelings towards him.