[@Shoryu Magami]

I figure that once Marche starts using his powers from Mateus and losing his soul that Cecil could actually help turn him to light in the long-run, due to his experience with darkness himself. (Even though Marche's case is much more severe) And him accepting to use both powers would probably be a part of that; Though if Mateus was to notice Cecil corrupting the mind of his pawn, he'd probably avoid having Marche confront him all-together. But a few encounters just might end up being too many; Though Mateus could probably remedy this by sending Marche after the others, claiming them to be 'practice' to take down a possible rival.