Jack is unusually tall and sharply built, with the long, almost jointless proportions of a marionette carved by someone who admired spiders. His grey skin has the dry, fine-grained look of moonlit slate, and a faint sheen of condensation beads along his temples whenever he crosses between worlds. Spikes of white hair rise from his scalp like clustered icy stalactites, each one carefully groomed into a different angle. His pupil-less eyes continually cycle through colors, not smoothly but in abrupt, unsettling changes, like lantern glass turned in a dark room. He drifts rather than walks, robe hems and vapor cloak lagging behind him as though the air has its own tide. For a man who rules a traveling fair, he is unexpectedly fastidious: each fingernail is painted with a tiny, perfectly centered black crescent.

Jack is generous with necessities and exacting about promises; he can spend an hour finding a hungry stranger supper, then refuse to bend a single clause in a bargain. He enjoys spectacle but distrusts anyone who mistakes it for foolishness. With workers and children he is patient and surprisingly direct; with nobles he is wry, formal, and watchful. When tired, his humor turns sharp, though he notices and apologizes once the moment has passed.

Jack is the eldest serving child of Ceredwen, Witch Queen of Lunasara, and the Lantern King, a powerful archfey who conceived him while weakened and trapped in humanoid form. His parentage made him a prince, but never an uncomplicated heir: the court regarded him as proof of the Lantern King's vulnerability and a living reminder of Ceredwen's bargains. As a young adult, Jack was sent to negotiate a truce with a mortal settlement that had trespassed on a fey crossing. He arrived prepared to win with riddles and threats. Instead, he found the settlement starving after a failed harvest. He traded away his first royal title to feed them, then spent years rebuilding the agreement into a traveling fair that could bring food, wonder, and knowledge to isolated communities. That fair became the Dark Moon Fair, appearing at appointed times across the Material Plane and other worlds. Jack now serves as its ringmaster and negotiator while remaining Prince of Lunasara. His relationships are strained but not severed: Ceredwen wants him back at court to settle a succession dispute, while the Lantern King alternates between proud gifts and cold tests of Jack's loyalty. His oldest mortal friend, a retired ferryman named Etsu, still sends him blunt letters about the people the fair has left behind. The current problem is that some fairground bargains have begun collecting a price Jack never agreed to, suggesting that someone is altering the fair's contracts from inside.