Joker functions as both Circus Master and Prison Director in Wonderland, embodying two distinct entities: White Joker and Black Joker. These are separate beings, not a single individual with split personalities. White Joker displays a polite and modest demeanor, typically appearing in circus ringleader attire. Black Joker exhibits a foul-mouthed, condescending, and violent nature, frequently associated with the prison warden role. Though sharing occupations, they maintain separate preferences and responsibilities.

During April Season—the season of lies—Joker arrives with the circus, triggering cyclical seasonal changes across Wonderland: spring at Heart Castle, autumn at Hatter Mansion, winter at Clover Tower, and summer at the Amusement Park. He wields power to manipulate these seasons and invade dreams, trapping targets like Alice within psychological illusions. His prison realm contains truths about Alice's past; he exploits her guilt by creating visions of her imprisoned and deceased sister, Lorina.

Joker antagonizes other roleholders, viewing Ace specifically as a "chess piece." He is universally distrusted, with characters such as Peter and Ace openly avoiding him. His influence extends beyond the Country of Joker; within the Country of Diamonds, he kidnaps Alice via a dream train, necessitating intervention from others. Every Wonderland inhabitant possesses a personal Joker, which must be acknowledged—consciously or subconsciously—to manifest.

In broader media, Joker appears as a main love interest in manga like *Junk Box Stories* and *Heart no Kuni no Alice ~Wonderful Wonder World~*, and as a prominent side character in the *Nightmare Trilogy* and *Circus to Usotsuki Game*. He features in the *Joker no Kuni no Alice Deluxe Edition Drama CD* and the film *Heart no Kuni no Alice*. His role often involves manipulating events across parallel worlds, as established in *Clover no Kuni no Alice*.