Kamelion played by Gerald Flood
Also played by Janet Fielding, Peter Davison, and Anthony Ainley
Kamelion was an android that was able to disguise itself as anyone it's controller wanted it to be. It was found by the Master who controlled it until it joined the TARDIS to escape the Master's influence.
Kamelion only appeared a few times, despite being aboard the TARDIS. Kamelion was killed at his own request by the Doctor.