Koharuko Koiwai

She is presumed to be the younger sister of Yousuke (Yotsuba's father), and therefore Yotsuba's aunt. She refers to Yousuke as onii-chan, which means "older brother". She lives in Tokyo and does not interact with the Koiwai family very often. Yotsuba described her as "someone who often bumps into a corner with her hands or feet". She seems to be not so good at dealing with children as she constantly keeps a serious expression on her face and is direct with her words. She often gets confused with Yotsuba and Yousuke's antics. [source: http://azumanga.wikia.com/wiki/Koharuko]