Tsubaki is a miko swordswoman. She was first hired by the yakuza boss in order avenge his earlier defeat at Mamoru's hands. She is pursuing them for the sake of justice. After finding out the truth about the yakuza boss, she takes revenge on that boss. She also decides to enter Mamoru's school. After Mamoru cheers her up while she was depressed about being old-fashioned, she begins to develop feelings for him.

(Taken from Wikipedia)