The fact that she's the only companion that can be turned into the mindflayer
By the way, a genuine question since I've never went the mindflayer route for me or for companions: is she the only one who can be turned, or is she the only one who explicitly offers? I think someone mentioned somewhere that the other companions also could be chosen in the correct circumstances (if you talk to [spoiler] as them, maybe?), but I obviously never checked.
She is the only one that can be turned. Other "can" be, but only if you're the one playing as them which makes it no different than turning your Tav into the squid, except there are some special dialogues like Mystra being able to bail out Gale. Karlach is the third option if you don't want to sacrifice MC or Orpheus
