The one on the left is Vergil, Dante's older twin brother. But that's how Vergil looked when they were 19, and they're about 28-29 in DMC4. Vergil is the second owner of the Yamato (first is Sparda), and if you played DMC1, you would fight Nelo Angelo, which was Vergil transformed by the Princess of Darkness. In DMC3, you find out why he turned into Nelo Angelo, as he was sent to hell by Dante when they fought because Vergil was greedy for power.
Also, there's a bunch of speculation that Nero's arm may have been possessed by Vergil's soul, but I think that's just silly. =D