The Doctor, otherwise known as the Professor, is the Head of a secretive, militant Quincy group known as the Schwarze Kreuz. They are nemesises of both the Soul Society of Japan as well as the Quincy Order. Secretly, however, the Doctor provides intelligence, technology, and weapon plans to the Commandant, to further the means of cleansing the World of the Living and the Spiritual Realms of all that is not True Quincy, readying his own force to serve him when needed.
He's been said to have been responsible for a Quincy massacre fifty years before the modern date, what was called the Pale Screams Genocide. Utilizing specially Genetic Modular Souls, the Doctor made sure to target a certain genetic strand he was convinced that could undermine the coming future True Quincy what he would call the Apex Predators of the Quincy race. He narrowly escaped death from the hands of both Hiryū Ishida and Taigen Funzen before disappearing back in the shadows, rarely heard from again as he awaited silently to bide his time for the next cleansing war.