When playing a companion in game I try to match whatever the MC is for world state, choices, etc if I'm coming in after them or if they app soon after my app into the game (and try to be vague about Warden/Hawke/Inquisitor if I don't have one - which is pretty easy, it's rare to find someone who deviates from a standard for those three). I generally ask first if someone is okay with being "(My Character)'s (Protag)" and while I've never gotten a no I'd roll with it if I did.

If they app in significantly later after I do and there's things I established that can't be easily retconned after they join (Or the prevous one dropped and a new one apps) I just roll with 'Oh, different protag'. Alternately I do this if I want to play a character from a very specific set of player controlled circumstances that pretty much never show up. Isabela who was given back to the Qun, exiled!drunk!Alistair, Cole who ragequit the Inquisition.