Leon is a fictional character, mostly appearing in Horror Maps
He is a bald dark figure that appears randomly to mostly appear as creepy or scary which for the most part doesn't work.
Leon is usually a random dark figure that Vanoss and his friends name when they see a dark figure in a Garry's Mod Horror map. Marcel once claimed that Leon was his butler. Leon kills, scares, or vanishes away from Vanoss and his buddies whenever they encounter him . In the sixth installment of GMod Horror map, Leon can be heard laughing. Leon's current status is unknown.
He once on his vid on 2014 gmod scary map said "i've never seen leon unless its sunny outside" so i think he might be refering to sunny leon.
Marcel claimed that he saw Leon in Vanoss's most recent Scary Map (which was actually scary for once), but they couldn't remember his name until later on in the video.