as someone who loves Karlach Id say bittersweet ending is a strong and emotional ending. The only thing Id add is a short stroll of us - player and her - in Avernus or her visiting us as "vacation". But a complete way to save her would feel like we forced developers to bend their vision to our caprice. I prefer to respect the way developers decided to write their story, meanwhile I also dont like how some characters I saved without even trying hard, but the one I cared the most had no way to be saved. I was ready to give up almost anything for her to stay and be happy
