Mazzy -- but I chose her because her "valiant servant of justice and righteousness" self would be the hardest sell. Still workable, but it would require godlike charisma.
For other votes, I think Anomen who's romancing Charname would be easier to convince to tolerate Brythe than he would be without romancing, Jaheira (already covered) and Aerie would probably be harder to convince than they would be without the romance (though Aerie might be able to accept the "I am a GOOD and NOBLE guy and I shall prove it by redeeming this piece of moral gutter filth! w00!" angle -- dunno what Brythe would say to that).
I think Charname with enough mad diplomacy skillz would be able to convince just about everyone to accpet Brythe, but it all depends on how Brythe acts, really. If he's hostile and threatening when the NPCs try to talk to him, then the more sensitive (Nalia, Aerie) or proud (Jaheira, Anomen, Mazzy)
NPC's would be much less willing to accept him, while the more patient (Keldorn, Cernd) or thick-skinned (Valygar, Imoen) would be more likely to keep giving him another chance. If he's more . . . normal is totally not the word to use here, but I'm blanking, so whatever, then people would still have doubts, but it would be easier for Charname to keep everyone happy.
(I put Keldorn down as patient, even though he's not too terribly, because he's neither a "let the gods sort them out" nor "KILL EVIL NOW" type -- unless the evil happens to be a drow, but that's just 'cause he's racist, the big baby.)