" . . . . neither one of you can cook anything except instant noodles ! "
Youko is Akira Tsubaki's elder sister. She is seven years older than him.
She considers it her duty to raise her brother properly in place of their deceased mother, and refuses to consider dating or getting married until he graduates and finds a job. She spends her time at home doing housekeeping.
Without knowing that he has a girlfriend, Akira's occasional strange behavior from the effects of Urabe's saliva sometimes makes Youko worry about her brother. Youko has met Urabe, but has not been told of the full extent of her relationship with Akira. Urabe said she was only a classmate Akira helps often at school, though some dialogue suggests she knows the truth.
It is strongly implied in the manga that Youko knows, since at least middle school, that she has the potential for being able to form a drool bond with another person, but she has chosen not to do so. She had a boyfriend in high school, but never formed a drool bond with him, probably because he could not form such a bond with Youko, or that she never attempted to do so. It may be that she views the drool bond as a genetic defect that she is ashamed of. Youko probably knows that Akira has the same ability to drool bond, but it is unclear if she has ever mentioned it to him, or if she knows if or how he became addicted to Urabe's drool.
From the picture below, Youko would wear her hair in a Twin Tail style during her high school years.