This is a tough one. Black Zetsu isn't technically a human, and it's debatable whether he's even a true living being, though he was in the Akatsuki as part of Zetsu. Madara thought Black Zetsu was a manifestation of his will, but in reality, Black Zetsu was a living manifestation of Kaguya's will, created by her before she was sealed away as the conduit through which she could return. Black Zetsu essentially manipulates the Uchiha to the point that Madara thinks the Infinite Tsukuyomi is something he discovers, only to have Black Zetsu betray him and reveal that Kaguya was pulling the strings all along.
Therefore Black Zetsu is either Kaguya's creation or Kaguya herself or, in all likelihood, both. As her creation, he's kind of like a brother to Hagoromo, which relates him to Naruto more directly. As Kaguya, though, he's still related to Naruto, just one more generation back. So either he's Naruto's super-distant uncle or his great-great-whatever-grandmother.
