From TARDIS Index File, the free Doctor Who reference.

The Master
Also known as:	{{{alias}}}
Race:	Time Lord (Android body)
Home Planet:	Gallifrey
Home Era:	Rassilon Era
Appearances:	WC: Scream of the Shalka PDA: Scream of the Shalka WC: The Feast of the Stone
Actor:	Derek Jacobi

The Master who traveled with the Doctor had an android body limited to functioning within the Doctor's TARDIS. Therefore he could not leave it. In effect, he had become the Doctor's companion.

Profile

The circumstances behind the creation of this version of the Master remain undisclosed. The Master himself was less than happy with this arrangement, saying that it was "an offer I was foolish enough to accept". He also said that he "was of aid to the Doctor after he had lost — during the events that so damaged him".

This Master retained his charm and his wit, together with his hypnotic abilities. He attempted without success to hypnotise the Doctor's latest companion Alison Cheney, but was interrupted by the Doctor. (WC: Scream of the Shalka)

Behind the Scenes

Derek Jacobi, who voiced this version of the Master would also later play Professor Yana, unwittingly, an Human incarnation of the Master in Utopia.