Could Vincent Valentine be Sephiroth's real father? There is a lot of evidence that supports this theory, and plenty of information to at least call serious doubts on Hojo's claim to Sephiroth's parentage.

The first piece of evidence is circumstantial in that Vincent *could* have been Sephiroth's father. Vincent is old enough to be Sephiroth's father (he was 27 at the time Sephiroth was born), was present around the time Sephiroth was conceived, and knew Sephiroth's mother very, very well for an extended period of time. It is heavily hinted at that he and Lucrecia were having an affair while she was still married to Hojo, something I believe to be the case. Therefore, if Lucrecia was with two different men and then became pregnant... I'm not so sure why she didn't consider that the child might be Vincent's, or perhaps she did and decided not to act on the worry, wanting to believe it was her husbands or afraid of what he might do if he learned otherwise. I don't think Vincent ever considered this at all, before Sephiroth was born or after he awoke in the coffin. It just seemed never to cross his mind, as if he did not think he could possibly be the father despite his involvement with Lucrecia. Hojo, too, is quite confident that Sephiroth is his son, even though I think he knew or at least heavily suspected that Lucrecia and Vincent were involved. Perhaps he was arrogant, as if to say the child *had* to be his because of who he was, believing himself to be much more important and intelligent than someone like Vincent. None of this is proof that Sephiroth is Vincent's son, no, but I think it does show that such a thing is in the realm of possibility simply because of the time frame and events that were going on. If Lucrecia was with two different men, it's almost a fifty-fifty shot, whether any of them realized this or not.

A second point is their appearances and personalities. As you can see above and in the game, Sephiroth and Vincent look very, very much alike. They have the same expressions, the same facial features, similar builds, and even similar personalities if you look at Sephiroth pre-Nibelheim. They say depression and other such personality traits can be inheritted... did Sephiroth inherit much of his moodiness and isolation from Vincent without ever realizing it? It is true that Hojo must have had some mental problems as well, but Sephiroth seemed more troubled than sick, much like Vincent. If you changed Sephiroth's hair and eye color to match Vincent's, or the other way around, the two would be nearly identical twins in appearance. But Hojo is short, rather unattractive, and slight in stature, appearing to be fragile, pale, and weak even in the flashbacks when he was younger. Vincent , on the other hand, is tall and leany muscular, just like Sephiroth. Sephiroth seemed to be brilliant as well, and that didn't neccessarily have to come from Hojo. His mother, too, was a scientist, and I don't think Vincent is a dim bulb, either. Sephiroth was much more likely to inheritt his good looks and physical skills from a former Turk like Vincent than from a scrawny little, near-sighted scientist like Hojo.

So how could Hojo not know this, since he was experimenting on Sephiroth all his life, even before he was born? I don't think what Hojo was doing, injecting Mako energy and Jenova cells into Sephiroth while he was still in the womb, would have involved looking at his DNA for parentage. And even if he did come across that information, why would Hojo have his own to match it with? Hojo just felt the child was his, hands down, and never thought to test to make sure. He wouldn't have discovered otherwise unless he was looking for it.

In conclusion, I think that Sephiroth being Vincent's son is a very real possibility, and an intriguing one as well. Think of the possibilities here... all the differences this could have made. While there is no mention of this possibility in the game, even though it makes very good sense and seems like an oversight not to at least bring up once, I strongly believe that Sephiroth could have been Vincent's son, a fact that could have changed both of their lives and Lucrecia's completely had someone known.