Rallon

Rallon attended the Prydonian Academy at the same time as the youthful First Doctor. Like the Doctor, he belonged to the the Deca. The Doctor and Rallon found the Celestial Toymaker together. The Toymaker took over Rallon's body and used it as his own, but during a confrontation with the Fifth Doctor, as Rallon's body began to decay, Rallon was able to expel the Toymaker by triggering all twelve of his regenerations at once, Rallon passing on while his Watcher remained to keep the Toymaker in check in future. (PDA: Divided Loyalties)