I have a new theory, when some demons die they become devil arms right. Well what if vergil did that and the devil brimger is vergil as a literal devil arm, it would have his soul. That explaims nero hearing the voice saying give me more power and might explain the sparda connection. Since.the db bonded.with nero due to an imjury maybe the blood was mixed? or maybe he is vergils son and that is why vergil sought him out.
Only an arm should not be able to contain a soul. At least, it doesn't seem logical to me. Also, Nero's Devil Bringer is part of his normal Devil Trigger in the concept art. Aside from that, it's red and blue -- if it's Vergil, why isn't it just blue?
Nero heard a voice echo ''power. Give me more power.'' That's obviously Vergil, but I think what happened is:
Nero was attacked in Mitis Forest, where there was a demon outbreak according to the DMC4 booklet. His arm was scratched, and the Devil Bringer resulted from it. At that point, when his demonic power was triggered, he could hear Vergil cry out from inside the Yamato in Agnus's lab.
So I think the Devil Bringer is simply a Devil Trigger arm. Nero never got his own Devil Trigger because he never got hurt badly. When he finally *did* get hurt badly (in Agnus's lab), he asked for more power. The Yamato responded by lending him Vergil's soul as a Devil Trigger. That's why it's blue, too.
As for why Nero even has the Devil Bringer, beats me. Capcom originally wanted Dante to have it, so I guess it's just a gameplay gimmick that turned into a new character they originally didn't intend to create. The story was just supposed to be about Dante, which makes me sad. These plotholes could've been avoided so easily.
True, but that's also why he tried to get Sparda's power. He probably thought that if he could obtain his power, he would somehow become more fully demonic.
Honestly, Nero being Vergil's son... I can actually easily see it happening. It's just that all the references to Sparda in DMC4 and how much Nero is like him wouldn't really make sense.
That said, if Nero is Vergil's son, that doesn't necessarily make it impossible for him to have some part of Sparda within him. There's some references to Nero Angelo too, which is interesting, since Nero Angelo is a corrupted Vergil, made to resemble Sparda. His theme song is even called Ultraviolet, which is popularly interpreted as a kind of purple or combination of purple and blue.
Argh, DMC5, please clarify it all. I'll bet Capcom said ''haven't you figured it out yet?'' to keep us speculating until a big reveal in DMC5.