I don't have one set theory, that's the problem. ^^; The only one that remains the same is this-Nero is a descendant of Sparda.
I can see him being either Vergil or Dante's kid, but more the former than the latter (I imagine Dante would at least suspect Nero was his son if it were the latter, and thus, would have acted accordingly in DMC4).
I can see him being a younger son of Sparda, and I can even see merit in him being a lab experiment-though I don't feel strongly about this one, because I don't believe he would have developed the way he did with the white hair, his personality, or his specific powers, without already being of the Sparda lineage.
So, I shall delve into my theories in more detail (I have done this a lot...I'm sure some are sick of seeing it, by now). :laugh:
Theory 1: Vergil is Nero's father
I figure Vergil would have wound up with someone within the year leading up to his meeting with Dante in DMC3. Perhaps he loved this woman, or perhaps he was merely being pragmatic in his approach, and wished to pass on the Sparda bloodline, having already made plans to raise the Temen-Ni-Gru. In this case, he likely knew he wasn't coming back, or was at least making arrangements in the event of his demise. Whether for love or for power, he would have wanted to pass his genes on.
Theory 2: Dante is Nero's father
I'm not leaning toward this one as much; while the idea of him being his father isn't a bad one, it just doesn't seem as likely. The vibe I get from Dante toward Nero during the events of DMC4 range between an older brother messing with his younger one, or that of an uncle. But, for the sake of argument, let's say Dante is his father. It could happen any number of ways, with any number of women. Though we never really see Dante with other women besides Lady, Trish, or Lucia, we don't see him with them romantically (though we imagine it quite a bit, lol). Chances are, if he's Nero's father, Nero would have been conceived roughly around the same time as he would have been if Vergil had been his dad; while Dante was young, brash, and hot-blooded as all hell.
But like I said, I don't get the "daddy" vibe from him in regards to Nero. He feels like an older relation, a mentor taking an otherwise orphaned younger relation under his wing.
Theory 3: Sparda is Nero's father
With this theory, no, I don't believe Sparda would have been unfaithful to Eva. So, for this to work, it would have to be with a woman perhaps a decade or so after her death. Now, in order for this theory to work, Sparda can't have died when Eva and the twins suspected (around their eighth birthday). He likely would have been horribly wounded, and couldn't make his way back to them due to that, and because he'd have to lay low. Now, skip forward to the passage of time; he resurfaces, but in a "Witness Protection" kind of way; that is to say, no one knows he's back, or perhaps only a select few do. He learns of Eva's demise, and believes his sons to be dead as well. Again, for this to work, we'd have to assume that he's somehow not getting information on current events and thus, doesn't know about either of his sons exploits; perhaps he's even been mislead by someone into believing his sons died during their battle.
Anyway, after some time grieving, perhaps he finds comfort with another woman-for the night, or for the rest of her natural life (which may very well have been cut short; I'll get into that in a minute). Nero is conceived from this union; the mother dies, either in childbirth or by some radical movement because she had a devil's child, who knows what becomes of Sparda, and Nero is taken into Credo and Kyrie's household (assumedly by their parents, in which Credo takes over as head of the household when they pass, and continues to guide Nero).
I'm not thinking this theory is as likely either, based on Nero's devil trigger and his arm. Instead of being half devil like Vergil and Dante, he seems to be more part devil (1/4). So, if not the son of either twins, and not Sparda's, I would think a grandchild (along the OP's lines of thinking).
I won't even bother with Theory 4 (experiment), because while I may accept some of the elements of it, I would only be able to do so with him still being of the Sparda lineage in mind. His personality is too much like both Dante and Vergil, he wears both of their colours (combined making Sparda's colour), and Sanctus himself said that he carries the blood of Sparda. So no, as far as I'm concerned, he wasn't some outsider that just happened to gain powers similar to theirs, who just happened to gain Sparda's blood through experiments.
Furthermore, if the Order had been the ones responsible for experimenting on him, wouldn't they already have known about his powers, about why his arm transformed when attacked by a demon (just before the events in DMC4). Agnus was far too surprised by the news, and if Sanctus had known, he would have coerced Credo into giving him up long before then.
Well, those are some of my theories; I lean more toward him being Vergil's son, really, despite all the reasons people have to believe otherwise (some of which are just downright silly; I've said it a dozen times or more, and I noticed you did, too; just because Vergil doesn't seem like father material, doesn't mean he isn't capable of being one. There are tons of people who shouldn't be parents, and it didn't stop them from procreating).
