Peri Brown

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

Biography
She traveled with the Fifth Doctor, and later on the Sixth Doctor. (TV: Planet of Fire, The Twin Dilemma; AUDIO: He Jests at Scars...)

A Life that Might Have Been
Had the Valeyard not won and killed the Doctor in the Matrix when merging with him, Peri would've traveled with the Doctor again at a later point. (AUDIO: He Jests at Scars...,The Widow's Assassin)

The Valeyard’s Mistake
After the Valeyard accidentally killed the Fourth Doctor by accidentally time-ramming him, the Fifth Doctor was erased from existence, and thus, Peru’s first meeting with the Doctor was altered or Temporal dissolution. (AUDIO: He Jests at Scars...)