Peri Brown

In a parallel universe, Peri Brown was a companion of the Fifth Doctor and Sixth Doctors, and would've rejoined the Sixth Doctor at a later date.

Biography
After the Valeyard accidentally killed the Fourth Doctor by time-ramming him, the Fifth Doctor was erased from existence, and thus, Peri's first meeting with the Doctor was altered or dissolved. (AUDIO: He Jests at Scars...)