Nero is the main protagonist of Devil May Cry 4, whom Sanctus makes multiple references to being "a descendant with Sparda's blood". He's a Holy Knight in the Order of the Sword, a religious order that worships Sparda and fights to protect the world from demons, also fighting to protect his childhood friend and love interest kyrie.
He is later revealed in DMC5 that he is the son of Vergil, as he has an innate resonance with his father's sword, the Yamato, though there are multiple sources from Capcom, including Hideaki Atsuno, that confirm Vergil's paternity over Nero that wasn't incorporated in DMC4's story for flexibility prior to the game's release.
