Rallonwashatellaraw was a Gallifreyan from the House of Stillhaven who attended the Prydonian Academy at the same time as the First Doctor. Like the Doctor, he belonged to the the Deca.
The Doctor and Rallon found the Celestial Toymaker together along with Millennia, another member of the Deca and Rallon's lover. The Toymaker took over Rallon's body and used it as his own, capturing Millennia as his servant.
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 and Millennia passing on while his Watcher remained to keep the Toymaker in check in future. (PROSE: Divided Loyalties)