It shouldn't just be Karlach
This post is made in a light-hearted tone. However, it is also accurate.
I'm talking about becoming a mind flayer. Infamously, someone needs to become a mind flayer at the end of the game unless you side with the Emperor. It can be Orpheus, the player character, or, if present, Karlach. It's honestly already weird how it is in game, because in most dialogue options she won't actually offer to do it, she'll just be an option by speaking to Orpheus as her. Whatever. Do I think Karlach would do it, considering she's facing certain death regardless? Yes! But she wouldn't be the only one. I'm gonna list the companions, and the likelihood of them offering to become Illithid to save Orpheus and you:
Wyll: this man has absolutely no sense of self-preservation. He is literally destined to become a mind flayer at the beginning of the game and he's just chilling with some tieflings (unbothered king tho ngl). He also will gladly give up his soul to eternal damnation for a man who doesn't love him. He's generally speaking extremely heroic and honestly, he'd offer before Karlach. Also, if he did turn, that should break his pact immediately due to him not having a soul anymore. So it could be a win-win, sort of.
Karlach: she is the only one who will actually do it in the game, and there's a reason for that. She has like two hours left to live anyways. She's super heroic. Let's move on.
Lae'zel: obviously, there is no fate she would hate more. Similarly to Orpheus, she would need to be convinced not to kill herself after killing the Netherbrain. However, I do think a Lae'zel who dedicated herself to the cause of freeing Prince Orpheus and liberating the Githyanki would be willing to commit the ultimate sacrifice for her cause. This might be an option only after a persuasion check though. But I think she would do it, considering her preferred option is for you to do it. Love her or hate her, Lae'zel cannot be called a hypocrite.
These are the three companions who I think could be convinced to become Illithid to save Orpheus. But I will quickly list the others, in no particular order, and give each a quote to what they would roughly say if asked.
Shadowheart: "Nah, I'd rather the Gith do it tbh. Who is he again?"
Astarion: "Lol no, darling."
Gale: "I would rather explode if that's alright with you! 😁"
Minthara: "Illithids might have a strange beauty to them, and the extra power is tempting, but this man is literally offering. Who is he again?"
Minsc: "Boo thinks it's not a very good idea. As for me, I'm still not sure why we're a fighting a giant brain. Or how it became giant in the first place."
Jaheira and Halsin: "🤨. Are you running a fever? How would I even...? Um, no, pup/my love/my friend."