It never ceases to amuse me how from time to time someone who actively dislikes Karlach comes here to chime in. LOL
I wonder if they'd feel the same if we swapped the questline of SH with Karlach's. I bet he'd throw the biggest tantrum.

Anyway, I see quite a constant fan movement of Astarion, as other's said, and followed (not closely though) by Karlach - but I don't frequent some mainstream SM platforms where opinions might differ. Also, my algorithm knows me too well.

Fact is that Karlach is very, very likeable for a majority of people, and her personality does bring a nice change of pace to the companions. I had no idea who she was until I was already 80h in the game and then she popped up and I was like ...WAIT, WHERE HAVE YOU BEEN ALL THIS TIME and she's be with me ever since.

Rawr.