Dr. Ichigaki created the Verrucae and surgically attached them to the backs of the three students of Mitamura he decieved into volunteering for the procedure. Yen, Ryo, and Kai were unaware of the controlling powers of the Verruca until it was too late and had their free will sapped as a result. The Verrucae functioned as second hearts and brains to those on whom they were implanted. Removing them would kill the wearer.