Katy Bell was a political science student. She was a Goth. Her father Douglas Bell was the local vicar. She helped the Sixth Doctor investigate how to stop the Carrionites. She followed the Doctor onto the TARDIS and realised that the words have power. She poisoned the Doctor because of the necklace she wore and stole a book from the future from the TARDIS. She was a conduit through which the Carrionites wanted to come through, so burnt the book she stole which stopped the Carrionites but killed her in the process. (AUDIO: The Carrionite Curse)
The Sixth Doctor's companions from stage plays are difficult to assert with certainty, as different actors played the Doctor in the same play. If a medium is not mentioned, then this incarnation did not have companions who were original to that medium; it does not mean that this Doctor failed to appear in that medium.