Yup, the Rogue will have a scene eventually. For the dark XP bottle, you should already be able to use that when you battle the knight. It sets his level back to 1
Speaking of the rogue, when you have the spirit inside of you and it's the rogue without the spirit, when he says "A certain word you need to say while meditating in the temple of pure light", what would the word be, or at least give me a hint.