水無瀬 由起子

First presented to be his stepsister, she is actually Minase’s live-in cousin and guardian, with the two acting like siblings due to them being close. She’s a strong-willed person, and enjoys tormenting Minase as well as teasing him at home with flashy outfits. Since she learned Latin and French in grad school, she helps Minase translate the contents of the book. [From Wikipedia]