The Portreeve was the title of the leader of the people of Castrovalva.
Although the Castrovalvans believed themselves to be a perfect democracy, the wizened old man whom they believed was their current, benevolent Portreeve was actually the Tremas Master, who had created Castrovalva and its people whole-cloth through Block Transfer Computation by harnessing the powers of Adric's mind. As such, the histories of Castrovalva were fake, and there had never been another leader, Portreeve or otherwise, than the deceitful Renegade Time Lord. (TV: Castrovalva)