10 Saddest Doctor Who Companion Departures
3. Bill Potts
Poor Bill. She was one of the fan favourite companions of New Who and its criminal that we never got more time with her. Bill deserved to be with Peter Capaldi's Doctor for at least another season. Bill and Twelve were up there with Ten and Donna in terms of popularity.
When Bill is injured - by injured, that means a huge hole shot through her - she's taken away to be fixed but only ends up being converted into a Cyberman. It's a heartbreaking story which sees Bill genuinely think she might get better and spend a long time in the presence of John Simm's Master who is manipulating her. What makes it worse, is that the Doctor misses her conversion by moments.
Still in typical Moffat style, Bill was brought back by Heather, her space puddle girlfriend (shhh, it's Doctor Who, don't question it) and turned into a magic space-travelling spirit and allowed to live on. According to the Twice Upon a Time novel, Bill and Heather ended up taking on a human life together and upon her deathbed as an old woman, Bill sent Heather back off into the universe.
Her story is one of tragedy and pain which took us on a rollercoaster of emotion and then got kind of confusing with the return and explaining away her magic survival.