Nero is the protagonist of Devil May Cry 4, sharing the lead role with Dante. An orphan adopted by the Sparda-worshipping Order of the Sword, Nero grew up to become a Holy Knight of the order. He does not get along well with others and prefers to work alone, so he is usually given the Order's "special assignments." During the first half of Devil May Cry 4 Nero thinks Dante is his enemy, but as the story progresses he realizes that the Order is his real adversary. Nero becomes a renegade, looking for answers and trying to save Kyrie (his childhood friend and love interest). Nero and Dante join forces to defeat the Savior, a flying statue possessed by a demon. They part on good terms when Dante entrusts him with a sword called Yamato (which belonged to Dante's brother, Vergil).
Nero wields a sword named Red Queen, a revolver called Blue Rose and the Devil Bringer, his demonic right arm. By defeating demons, Nero increases the Devil Bringer's reach. When Dante gives him Yamato, a Devil Trigger summons a devil-samurai avatar to fight with him. Like Dante, Nero has Sparda's blood; however, his relationship with Dante is explored only in the novelization when Dante notes that Nero resembles Vergil.
Source: Wikipedia, "[en.wikipedia.org/wiki/Characters_of_Devil_May_Cry%23Nero Characters of Devil May Cry]", available under the CC-BY-SA License.