One of my theories is that Nero was a clone of Sparda created by the Sparda cult. They had to use a human sacrifice to create Nero and that is why he is not a full demon. Also people have been saying that Nero could be Vergil and when we think about it, if we combined Vergil and Dante we would get a Sparda clone in human form, which in this case is Nero. So he has Dante's looks with Vergil's attitude.
Nero's arm (the Devil Bringer) actually looks like its from Sparda's Firing body aswel. If my theory is correct, then Nero has been raised from being a child by the Sparda worshipers. I think their reason for creating Nero was because they somehow knew that Dante would try to kill them all in the future and so they needed a bodyguard that could match him, and who better than a clone of his own blood.
Tell me your theories...