Joker is mysterious being in borderlands, he appears when the final game is cleared by Usagi Yuzuha and Ryōhei Arisu, as they are ready to be sent back to the real world after refusing the permanent residence stay.
When Arisu asks him if he is the ruler of all life in the borderlands, he replies by asking Arisu if he look like a God or a Devil. Arisu then replies that he's only an intermediator. A reference is then shown in the end of chapter 63 implying that Joker is the intermediator between the God and the Devil, ruling the borderlands.
(The borderland between life and death, hence the name)