Yukine is a second year student who hangs around in the library's reference room during lunchtime. She takes care of many newcomers from other schools who also happen to be delinquents — in particular, people who knew her brother. She's very kind, and avails Tomoya and Youhei an instruction book for magical spells for them to try.
Yukine is actually involved deeply with the story relating to the delinquents. Her brother is well known for being a delinquent, and this sometimes gets her into different situations involving the delinquents of the town. It is later revealed, in After Story, that her brother died and nobody knew that other than the group of people that reside on her brother's side. Yukine later helps reunite the members of opposite gangs, and they all end up being friends.