Jii worked as the famous magician Toichi Kuroba's assistant. He also knew of Toichi's alter ego as the Kaitou Kid, and presumably assisted him in his heists. According to Jii, Toichi made him swear never to tell his son Kaito about his father's secret identity. Because Kaito did not immediately recognize Jii, it seems that he may not have been close to the entire Kuroba family, or perhaps Toichi wanted to protect Kaito from his Kaitou Kid identity, and thus kept him away from Jii.
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A sweet old man