Kamelion

Kamelion was an advanced, shape-shifting android who was found by the Master on the planet Xeraphas, who could be controlled by anyone with a strong will. The android was a tool of a previous invader of the planet. The Master used him to replace King John of England in an attempt to disrupt history, but Kamelion was turned to the Fifth Doctor's side.

Kamelion remained relatively dormant in the TARDIS, until the Master took mental control of him anew, and used him against the Doctor on the harsh planet of Sarn. The Master did so because he had accidentally been reduced to a few inches in height thanks to a malfunction in his Tissue Compression Eliminator. Kamelion was eventually torn between his programming and the individuality he developed under the Doctor's tutelage, and had the Doctor destroy him rather than risk being the Master's slave again.