One-Eyed Willy is a small demon encountered repeatedly by Garcia Hotspur and his demonic partner Johnson as they travel through Hell in Shadows of the Damned. Johnson is an old friend of Willy's, describing him as "my aunt's first husband's adopted son from the Ukraine", which Garcia flatly describes as making them "practically brothers". Willy is nowhere near as aggressive or brave as the other demons encountered by Garcia, and in fact all Willy wants to do is be left alone to sleep in peace. In fact, despite him being accompanied by Johnson, Willy is so afraid of Garcia (whether it's because he knows he's a demon hunter or simply because he's a Human in Hell) that whenever Garcia nears him, the poor demon is startled awake, literally shits himself in fear, and flies away in a panic. Unfortunately for Willy, the spots he picks for sleeping are all along the path Garcia takes on his quest to rescue his beloved Paula from the demon lord Fleming, so poor Willy is going to be losing control of his bowels rather a lot over the course of the game.
Willy's feces burns brightly as though it were comprised of phosphorus, and Garcia comments that they can use this property to remember where they've been. This is because in terms of gameplay mechanics, One-Eyed Willy serves as the game's checkpoint system; whenever he shits himself and flees, the game saves. At one point Garcia asks about the fiery properties of Willy's feces, and Johnson merely replies that "You should see him piss."