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Joker is a mysterious being in the Borderlands, he appears with his signature Joker card, hence his alias, when the final game, Queen of Hearts, is cleared by Usagi Yuzuha and Ryōhei Arisu, as they have refused the permanent residency in the Borderlands.

When Arisu asks Joker if he is the ruler of the Borderlands, he instead asks Arisu if he looks like a God or a Devil. Then the image of Sanzu River comes to Arisu’s mind, believing Joker is the equivalent of the ferryman (who is unnamed but the role is similar to Charon in Greek myth) who carries the dead to the netherworld, hence his subsequent reply, an intermediator. Immediately afterward, all players refusing permanent residency are sent back to reality as promised.