Kamelion

Kamelion was a shape-changing android, a companion of the Fifth Doctor. Originally, Kamelion was a tool of the renegade Time Lord known as the Master. At the conclusion of the Master's last encounter with the Doctor, he was trapped on the planet Xeriphas. When the Master returned, he had Kamelion with him, calling it a souvenir he had picked up from there. Although Kamelion was sentient to a degree, it was also extremely weak-willed, and therefore open to manipulation by any strong personalities around it. The Doctor freed Kamelion from the Master's grip, and it joined the Doctor in the TARDIS ("The King's Demons").

Kamelion eventually fell under the Master's influence again during the events of Planet of Fire. In the end, Kamelion begged the Doctor to destroy it, and the Doctor reluctantly honoured that request.

The Expanded Universe
Kamelion is also featured in the spin-off novels The Crystal Bucephalus by Craig Hinton, Imperial Moon and The Ultimate Treasure both by Christopher Bulis. In The Crystal Bucephalus its absence from the televised stories is explained by saying that it remained in the TARDIS for fear of being taken over by a stronger personality and used against the Doctor. In The Ultimate Treasure it is stated that Kamelion was the product of a race known as the Gelsandorans. Some of Kamelion's personality survived due to interfacing with the TARDIS, and the Gelsandorans gave it a new body only for it to sacrifice itself to save the Doctor and Peri.

Kamelion also features in several short stories set in the Doctor Who universe, notably One Perfect Twilight by Craig Hinton (Perfect Timing), where the Doctor realises that obedience and slavery are built into Kamelion's makeup and The Reproductive Cycle by Matthew Griffiths (Short Trips: Life Sciences). In the latter story, the "child" of Kamelion and the TARDIS, who becomes a double of Peri, takes her place on Earth while the real Peri travels with the Doctor.