A particularly singular choice is Rachel Cooper who appeared in the 3rd Seventh Doctor story from the Klein Trilogy, Architects of History. Played by Lenora Crichlow (of Being Human fame) Rachel is unique (so far) within the confines of inconstant companions as she (within the narrative) was actually a long time companion of the Doctor, but we only encounter her the one time as she is from a parallel branch of history…
Why the acclaim?
The performance of Rachel Cooper was poignant – the character never gets to interact with the Doctor and is left pining for a real companions existence of travel, alien menaces and all the meat and drink of the Time Lord’s companion. Sadly success here is measured only by the outcome, that being the removal of her timeline! We never even know how her life turns out in the ‘correct’ timeline, nor if she even gets to exist!
Meanwhile Rachel fights the good fight and helps win the day all unsung except in the memories of we the listeners.
Chances of a return?
I think slim as the timeline she is from has been air-brushed from history. I believe fans liked her though so we never know!