Missy
Calling Missy a companion seems almost reductive, given that her relationship – friendship/rivalry/squabble/whatever you want to call it – with the Doctor has spanned the majority of Doctor Who’s run, in both its classic period and the modern day reboot. But the first female Master did indeed travel on the TARDIS for a bit in season 10, so companion is what we will go with.
The dynamic between Missy and Twelve was so fascinating during Peter Capaldi’s final season (made even more poignant thanks to his eternally gangbusters chemistry with Michelle Gomez). As Missy attempted to reckon with her many regenerations worth of destruction – and Twelve tried to figure out whether he could put any trust in her ability to change – the duo’s relationship shifted in constantly interesting ways. If only we’d gotten more time with these two together as real friends, rather than enemies! (Surely, we could have ditched one of the installments in the lackluster Monks trilogy for a Missy-focused installment?)
Best Episode: Season 10’s “World Enough and Time” and ”The Doctor Falls.” Yes, this is cheating by naming a two-parter here, but it feels like these episodes really need to be taken together, particularly when it comes to Missy’s role in the story. The season 10 finale sees Missy face off with her worst self in the form of John Simm’s Master, who is less than pleased with his later regeneration’s attempt to turn toward the light. That Missy dies before Twelve can ever learn of the fact that she did choose to stand with him in the end is honestly one of the season’s great tragedies and something I personally will be sad over for a long time.