Kamelion

Kamelion was an android discovered by the Master on Xeriphas and could use the voice and form of any living being. Kamelion's personality became a battle ground between the Master and the Doctor. Kamelion took on the appearance of King John, the Master, the Doctor and Tegan. The Master lured the Tardis to Sarn (were Turlough found his brother) and he used Kamelion again (who had joined the Doctor in the Tardis) and he begged the Doctor to destroy him.

Kamelion was in The King's Demons, Planet of Fire and as a memory in The Caves of Androzani: Episode 4.