In my opinion, Nero is a single character who just fits into the story. In the TGS trailer, Dante tells Nero to return "it" to me. The object may be the Amulet given to him as a child, either the Perfect Amulet or one of the halves.
As another note, the Devil Bringer may be a part of Nero. If he is human and his arm represents that of a demon, what could have his arm mutate like that?
Could it be that he acquired the Amulet and part of its demonic powers transfered into his system? Maybe he acquired Vergil's part of the Amulet and his power or soul transfered into Nero, giving birth to the Devil Bringer. Since he hears voices in his head, it could be Vergil who is talking to him. Anyway.
We will never know until the game is released. Much is left unanswered and only time will help us in this great mystery.