Kamelion

Kamelion was a shape-changing android, a companion of the Fifth Doctor.

Kamelion was the creation of Gelsandorans. (PDA: The Ultimate Treasure)

Kamelion was the tool of an invader of the planet Xeriphas. When the Master was trapped on that planet, he discovered Kamelion and used him in his next encounter with the Doctor. 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. (DW: The King's Demons)

Kamelion preferred to remain in the TARDIS for fear of being taken over by a stronger personality and used against the Doctor. (MA: The Crystal Bucephalus)

Kamelion eventually fell under the Master's influence again. In the end, Kamelion begged the Doctor to destroy it, and he reluctantly honoured that request. (DW: Planet of Fire)

Some of Kamelion's personality survived due to interfacing with the TARDIS. When the ship arrived on Gelsandor, the natives gave it a new body, only for it to sacrifice itself to save the Doctor and Peri. (PDA: The Ultimate Treasure)

Kamelion and the TARDIS had a child together, which was raised by the Doctor and Peri. It developed into a double of Peri and took her place on Earth while the real Peri travelled with the Doctor. (ST: The Reproductive Cycle)

Behind the scenes
Kamelion only appeared twice in the series; further appearances were made impractical by the special effects required. A scene involving Kamelion was filmed for The Awakening but was cut before transmission. As a result there are no references to Kamelion in any episodes between The King's Demons and Planet of Fire, not even in The Five Doctors.