Sly is a creature known as a Nocturnal. He is partially responsible for the abduction and imprisonment of Aggie James from an alley near Fern Street in San Francisco. He is also believed to be responsible for the murder of Paul Fischer, a member of the BoyCo gang. He normally travels with his partner, Pierre. As a Nocturnal creature, Sly possessed unique skills including the ability to drop to the ground and jump back up to rooftops without making a sound. He is also a proficient killer.
Aggie James was camped out in a cardboard box in an alley when he heard Sly and Pierre jump onto a nearby dumpster and discuss his abduction. However, he was too high on heroine to understand what was about to happen to him. He also overhears a conversion between sly and Pierre indicating that other Nocturnal's in their group were hunting gang leader Johnny Yee.
Bryan receives his first glimpse of Sly in a dream. The dream took place one block away from his apartment and consisted of Bryan and Sly hunting BoyCo gang member Paul Fischer. At this point Bryan doesn’t know Sly’s name, but he does glimpse Sly's horrible visage. When Bryan awakes, he rushes to the scene of the murder and he sees a flash of Sly escaping by jumping to the roof and slipping away.
Tiffany is a 67-year-old woman who lives in an apartment building to the left of the alley where Paul Fischer was murdered. She witnessed the murder from a distance and recounted her account to Homicide Detectives "Pookie" Chang and Bryan Clauser. Her description of Sly matched Bryan's description from his dream.
Rex met Sly, Pierre and Sir Voh outside his home and recognized them from his dreams and his drawings. He greeted Sly by name, which surprised the snake-like creature. The three creatures presented a beaten and bloody Issac Moses, the last of the BoyCo gang, to Rex while demonstrating awe and reverence.
The character is referred to with only the single name of Sly. Bryan referred to him as a "Snake Man".
Sly is described as a thick man sporting a dirty gray blanket as a makeshift cloak that barely covered his hulking frame. This skin of Sly’s pointed face was green and looked like a snake. He had razor sharp, neon white teeth and a forked tongue. Also, he has a low hissing voice that sounds like sandpaper. When Tiffany saw him, he was wearing a black trench coat and a baseball hat with the initials of "EFF" on the cap, in addition to the cloak.