NOTE THAT THERE ARE SPOILERS BELOW!!
So if you don't wanna be spoiled, stop reading right now.
Go back...
Still here? Okay. This is really just for fun, but it struck me today that all the companions we know about come with huge red flags which they constantly wave in our faces.
ASTARION: Vampire Daddy has inscribed part of a devil pact on the skin of his back, so Astarion could well be 'owned' by two different big bads. Plus, he doesn't really wanna be 'saved', just free, so if you're inclined to good, he's not your huckleberry.
GALE: Has a bomb in his chest and aspires to either get with or replace Mystra. He is the avatar of hubris in the game, his end is forecast to be dark indeed. Be careful that he doesn't drag you down with him.
SHADOWHEART: Whether she's mind controlled by Shar or willingly serves her, Shar is going to be a present and formidable opponent, and you can't count on Shadowheart's help in fighting against her influence, as she is serious and committed. There is hope that a good end awaits, but I doubt it will be easy.
LAE'ZEL: Cannot get out of Vlakith's shadow. Is a true believer, and will probably never care about you, even if you, you know, hook up and stuff. There may be a redemption arc here, but it's going to be a while before you find it, and if you play as a good guy, all you do is piss her off more and more. Doubt her idea of 'good' would match ours, anyway, even if she does reform. Yet her sense of honor and commitment to ideals are almost paladinlike, so if she could be turned... formidable.
WYLL: Sold his soul and now wants to get it back. He's completely owned. Can he break the shackles? Maybe, but it'll take a lot of work from you. At least Mizora isn't a goddess (see Shadowheart, Gale) or archdevil (see Karlach), so you've got that going for you.
KARLACH: An unwilling champion of Zariel who has an infernal engine for a heart. You'll need soul coins to keep her running, and you will run out. Be careful, because she warns you about what's coming. She's a sweet soul with an anger management issue, and you'll want to help her, but if you give your heart to her, prepare to have it broken. (Devs started talking about crying at a certain ending for one companion immediately after talking about her. You have been warned!)
MINTHARA: I mean... you gotta kill everyone at the glade to even get her, so... not a good ending. She is straight out.
You may notice the trend. In each case, someone or something has power over your companions, beyond the tadpole.