Looks like I'll hafta overtell that guys theory
1) The looks. In other words, why does Dante's twin look so younger. Because when he got possessed and trapped in Nelo Angelo Armor, he stopped aging. And look the age he was corrupted.
2) Why is he human? Because after Dante defeated him in DMC1, he sealed his powers and they’ve reappeared just recently as the Devil-right hand
3) Why is he doing this? He lost his memory after being defeated, the only thing he could remember was his name Nero/Nelo. That’s why he uses new weapons.
4) Why the hand? See No2. When touched the stolen Sparda’s sword/amulet, his powers reopened a bit.
If you look at the trailer of the gameplay (it’s available at the official site) you’ll see that he has an attack (accelerating forward and cutting those in his way with a horizontal slash) which is similar to Vergil’s Rapid Slash