Nero is too old to be Vergil's son. Dante is barely into his 30s in DMC4, and Nero is into his 20s, which means that Vergil would have had to had sex with someone when he was TEN to be Nero's father. Very unlikely, since that's when their mother was killed. Sanctus does say that Nero carries the blood of Sparda, so it's likely that Nero is some long separated descendant of the big guy himself. Therefore, he's not directly related to Dante and Vergil, but he's still a descendant of Sparda. But yes, Nero needs Yamato in order to DT, or an extremely stressful situation, which is how he DTed before Yamato flew to him.

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"Trust me. When I get in the fight, Devils will Cry."