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Yukio has Demon blood in him, so it was fairly ease for Satan to possess him, since Yukio was becoming more and more demon.

And I think you meant Shirō, the twins adopted father. Satan has said that Shirō was the only capable to be possessed by Satan, but that was because Shirō was that strong. Yukio wasn't as strong as Shirō, but his demon blood made it possible to be possessed as well.

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