Shunkan Heroism 

There's an urban legend about a hero that is able to take out entire groups of delinquents. People have only seen the hero's back, where there's an X upon the hero's clothes. A friendless yankee type girl wearing an X on her back, Kinami Shino, may be this hero. She forcefully makes friends with Batsumaru, a boy with red and white hair, dragging him unwillingly into the world of violence. But is Shino really the hero or are things a little more complicated? (from Mangahelpers)