Ace

In a parallel universe, Dorothy "Ace" Gale (or Gale McShane) would've been a companion of the Seventh Doctor, had the Valeyard not defeated him and merged with him in the Matrix.

A Life that Might Have Been
Unlike her mainstream counterpart, Ace would have travelled with the Doctor, and later on Thomas Hector "Hex" Shofield would have joined them. (AUDIO: He Jests at Scars..., The Harvest)