I think the most likely scenarios are that he's either a new character entirely, or he's Vergil in some regard.
In terms of that second possibility, the leading speculation is that it's Vergil's soul possessing a different body. If it's a human's body, this would explain why his gameplay revolves around summoning demons rather than fighting personally, since it's too weak physically to beat Demons head on.
As for the other Vergil that is actually his body, let's say that this is basically a soulless husk being controlled by the main villain, who retrieves Yamato from Nero's arm in order to open a tear into the Demon World and help nourish the Demon Tree. Maybe this Vergil is effectively a complete amnesiac, but sill retains part of Vergil's personality (Akin to Nelo Angelo), which is why he doesn't kill the defeated Nero.
I think this is far and away the most likely scenario based on what we currently know, and it makes sense if we consider the existence of the original Nelo Angelo to be that of Vergil's soul trapped within the armor by Mundus, which is the case as presented in DMC4. So effectively, in order to make Vergil his pawn, Mundus separated Vergil's soul from his body, leaving each incomplete and easier to control.