This is very interesting comment I found in different thread in this forum:
"Based on interviews and the art book, Larian didn't really know which direction to take Karlach. Apparently, she went through three significant versions. The aristocratic sharpshooter version who people through was an EA placeholder, the "Cool Karlach" who was a bit grimmer and tougher, and then the one we got, who was apparently pretty much based on the voice actress' charisma and delivery. Class-wise, they did not know whether she'd be a rogue (first version), sorcerer, paladin, or barbarian. Additionally, the general... oddities of the infernal engine idea (is it in her body, easily removed, why don't we ever see it? the cutscenes feel like something from 2010) make it clear it was a pretty late development. I think the voice actress has said she had no idea what the infernal engine was or that Karlach's chest was basically glowing."
