like others said, you don't need to give the commentary. just say "i'd rather not play with that specific character but i'd love to thread with your other ones!"

if they push for a reason, then you can gently say "i don't want to shit on something you like but i just personally really dislike the character, i hope you have fun with them but i'd rather play with other characters". any sane person will accept that and appreciate your honesty, and if they don't take it well than that's a red flag.