Larian Studios Forums Baldur's Gate III Baldur's Gate III - Story & Character Discussion [END SPOILS WARNING] Addressing Karlach (PLEASE PUT SPOILERS IN TAGS)

'redemption trap', she is twisted by her nature and could not be changed, but lures 'I can fix her' folks effectively into her romance and makes people doubt their morals in regards of her looks(crazy/hot scale is off the charts).

couldn't be redeemed by normal means because demons embody evil concepts and are born from the souls of sinners condemned to the Abyss, but her special case is that her 'soul' recieved divine intervention from a major good goddess and now she tries to overcome her demonic nature with the help of said goddess and MC. To nudge her a bit in the right direction or make her fall to her previous state is ultimately players' call.

a mutant born of lineage of humans influenced by a certain demon's poison which makes them ugly, short lived and 'a bit' feral. She lived a very harsh life under the city, fighting for survival. She learned to trust nobody and that it's safe to pledge allegiance to whomever seems stronger. In truth she is only loyal to herself and her idea of serving the strong to eventually aquire or steal their strenght for herself. In her romance she could be 'calmed down' and learn to trust people more despite her awful life experience, shifting her to true neutral aligment. She is the most complex and deep companion from my opinion because she hides her true motives well and her action suddenly become more human if you try to put yourself in her shoes.

you can't change her nature no matter what. However, game also implies that such cases are nothing short of micarles performed by the gods (Arue) and you can either accept her twisted being for what it is, or put her down for good. Her route is not about you influencing her, but about her influencing you, making you forgive more and more of her atrocities for the sake of love.