Marcus is a stocky, greying, handsome man who appears to be in his mid 60's. Tales and legends of the area of the Enchanted Woods put his age at least 10 times that. Certainly the tower and an inhabitant were present when the area was colonised and it figures in the occasional tales of the nomads. That the woods derive their reputaion from him is definite.
Marcus is well disposed towards adventurers. He is willing to offer advice and some help. His tower is extra-dimensional, well appointed and provisioned. For adventurers who are lost, finding Marcus or his tower can be a welcome relief.
Little is known of him. Most divination magic tends not to work on him and even the Powers are unaware of his True Name (or aren't telling). While he claims the dragon Rhalina is his wife, there is little actual evidence to support this. It may have been a convienient way to answer a difficult question.
Marcus' powers are also surmised and inferred, rather than known. It appears true that he has created a magical world or plane populated by sentient beings (some of which escape into Alusia) and occasionally engages the services of the Butlers. Some have guessed that maybe he is a god in hiding. But, given the Calamar assault on the tower, which was in response to Sabrina summoning the Bifrost, and their lack of interest towards him, this seems unlikely.