Personally Karlach is my favorite character, followed closely by Lae'zel. But if there was any reason beyond "this is what Larian wanted" for her to be the way she is, then it would be the fact that despite having been part of the game from the start, and apparently Larian being confident enough in her eventual inclusion to have her get a card in the BG3 magic the gathering set, she was one of the last characters to get worked on.
