Well, I got a lot of time to think about Nero and I think he is...
VERGIL!
Look, there are many facts that approve it:
1.Nero's voice is nearly similar to Vergil's.
2.Nero forgot his ass-kickin' style because the influence of Mundus.
3.Nelo Angelo is mistranslation. In fact he is called Nero Angelo
4.His Devil Bringer arm is more likely a piece of Nero Angelo's armor.
5.He remembers Dante and his style but doesn't remember that he is his brother.
6.Nero Angelo disappeared in blue flash. It doesn't mean he was dead.
7.He was found by the Order of Knights who gave him Red Queen and a handgun.
How's this?