The Brink of Death
Genesta was a Time Lord technician that seemingly tried to help the Sixth Doctor when he was only an echo of himself in the Matrix after The Valeyard took over his life.
She had access to a TARDIS.
Eventually, in her final encounter with The Doctor, Genesta revealed herself to be The Valeyard in disguise, leaving The Doctor uncertain of whether she actually existed, or whether it was a disguise of the Valeyard from the start. The Valeyard himself refused to confirm either way. (The Brink of Death)