Dante Redgrave
Son of Sparda
You can't say that the DB is for sure Vergil, because they did not tell use WHAT Nero's relation is. Sanctus says that Nero carries the blood of Sparda, meaning that he does have some kind of family relation to the twins, however distant. We were never told when the DB really started to develop on Nero's arm, though it's indicated that he's had it for quite some time considering how proficient he is with it in his first fights. He's obviously been hiding it for quite sometime, instead of having brought it to Credo's attention. In fact, the DB never once actually takes over Nero. The aura does HIS bidding, not the other way around. If it were Vergil, he would not be letting some human control him.