Honestly, the Vergil's son theory is the only one that isn't ridiculous. Most theories to refute it are desperate and based almost entirely on faulty assumptions about his character, and incomplete time tables on the matter. There is some merit to the direct similarities between Nero and Sparda, but nothing that reasonably answers his origins. Even the "Yamato serves as a remnant of Vergil's soul much like most other devil arms and this influenced Nero" theory is very false considering his arm changed long (and supposedly his white hair was never pointed out as being 'different') before any implied contact with Yamato occurred. The Vergil's son theory has been 'confirmed' twice now, so far it is the most valid as opposed to any others.
And the 'Nero is Vergil's son' theory is based on one assumption Sanctus makes in the novel.
When we have the choice between assumptions that make little sense, and assumptions that make more sense, I'll always choose those that make more sense. I did not make assumptions about Vergil's character... but if you were referring to somebody else, fair enough.
If Dante is around 30 in DMC 4 (and Vergil would be too), then Nero is not Vergil's son. According to the DMC 4 novel, Dante ages at the same rate as normal humans. Since he is 19 in DMC3 (which has been accepted by all forums and DMC fans I can find on the internet), and DMC4 was stated to take place about 10 years later (seems correct when you look at Lady), Dante/Vergil would be about 30.
DMC 4 hints at the idea that Nero is more connected to Sparda than to Vergil, and nothing indicates he is Vergil's son. Even the novel does not confirm it - just like DMC4, it only confirms he is related to Sparda in some way.
If you only want to listen to evidence, then Nero must be a human. The story says he is, so why doubt it, right? Too bad that's a very ignorant stance to take, all things considered. You can figure out he's related to Sparda (or has his genes) just by logical inference. No humans beside those of the Sparda bloodline in DMC have white hair, as far as we know... and in Japanese culture, it's a sign of demonic possession. He just happens to wear red and blue, happens to have extraordinary power, happens to have the same kind of personality as Dante, and gets a demonic arm out of nowhere? You see my point, right? The same kind of thing is happening here.
You're assuming he's Vergil's son, but you're telling me my ideas are ridiculous? I know I have no proof for him being an experiment or distant relative of Sparda's, but you have no proof he's Vergil's son either. You're just telling people they're wrong for having theories different from yours.
Also:
*- What does his arm have to do with the Yamato, or even Vergil? His arm has not been connected to Vergil, so I don't see your point... or are you making
assumptions? You know, the kind of assumption you criticized yourself. The game implies that Vergil's soul is released inside Nero once he takes the Yamato. Once he takes it, Nero literally says Vergil's 'power' line and gets a blue floating thing behind him. Blue is Vergil's signature color, and no other demon beside Vergil has had a sheath for the Yamato inside their arm. Nero needs the Yamato to go into DT. Without it, he has nothing. If his arm is connected to Vergil, he would not need the Yamato to go into Devil Trigger. He wouldn't even need it to break out of Agnus's lab.
- How has the 'Vergil's son theory' been confirmed twice? Nobody from the DMC development team confirmed it, and the novel didn't confirm it either. It only told us that Sanctus had a little thought... a thought that wasn't based on any evidence. If you have new information, I'd love to hear it. Otherwise... there is nothing that directly indicates he is Vergil's son. And that's why people have speculated for 32 pages now. Telling people they're idiots for speculating is not helpful. It's comparable to flaming, and flaming shouldn't be tolerated.