Ace

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

Ace would've traveled with the Doctor, and later on Thomas Hector "Hex" Shofield would've joined them. (He Jests at Scars... (audio story),The Harvest (audio story))

Sources:

* Ace Tardis Wiki Article.