The eponymous Doctor Sax, also part of Jackie's fantasy world, is a dark but ultimately friendly figure with a shrouded black cape, an inky black slouch hat, a haunting laugh, and a "disease of the night" called "Visagus Nightsoil" that causes his skin to turn mossy green at night. Sax, who also came to Lowell because of the Great World Snake, lives in the forest in the neighboring town of Dracut, where he conducts various alchemical experiments, attempting to concoct a potion to destroy the Snake when it awakens.