Rhythm Wily, AKA 'Hammer Girl', AKA Tetsu Oni, is a reploid of complicated origins. She was originally created by Dr. Wily to destroy Mega Man. Things happened, some good, some bad.

In the MMX timeline she was rebuilt once again by Dr. Cain to see if she was just a doll or one of the mysterious new beings that would be later dubbed Reploids.

Her past and current whereabouts are unknown, but some say she's searching for her family.