Little about him is known, except that he was raised in the village of Iwagakure as a Shinobi and taken in by Kyrie's and Credo's family since childhood, growing into young adulthood together. Nero got his name from a black blanket in which he was found. His actual origin still remains unknown but Sanctus makes multiple references to Nero being "a descendant with Sparda's blood."
Mentioned in an incident report, a number of fire creature appeared made by a fire jutsu suddenly from the Mitis Forest and attacked Fortuna. Luckily, Nero was present at the scene and exterminated all of them. During this incident, Kyrie, who was accompanying Nero, was injured while Nero received a minor injury to his right shoulder. Nero's sword, the Red Queen, was damaged and sent for repair. Kyrie recovered completely, but Nero's shoulder never healed, and he continued to carry his arm in a sling after the incident.
On the day of the Festival of the Blade, Nero barely manages to make it to Kyrie's performance in time after dealing with a group of shinobis, with a man in red secretly watching him from the rooftops. He leaves her a gift and then prepares to leave, having grown tired of Sanctus's preaching. Just before he can, though, the mysterious man in red bursts in through the glass ceiling during a prayer and slays Sanctus, the leader of the Order.
Before the assassin (who is later revealed to be Dante) can confirm the kill, Credo, along with a battalion of knights confront him, but are completely outmatched. Kyrie, seeing that Dante is going to kill her brother, finally escapes Nero's hold on her and runs to them, but is knocked down in the process. When Nero sees the man in red looming over Kyrie, he kicks him in the face and orders Credo and Kyrie to go get help while he stalls Dante.
Fight of Destiny
The two fight, and Nero is forced to reveal his Will of Rock. With it, he is able to knock Dante back and impale him with his own sword on Sparda's statue. But Dante frees himself and jumps down, to which Dante then pulls Rebellion and points out the knights he had slain, who turned out to be fire creatures made by his fire jutsu. Confused, Nero watches as Dante flees the scene.
Shortly after, Kyrie returns Nero's sword, Red Queen, at Credo's request. Credo gives an order to Nero - to pursue the man in red, with witnesses seeing him heading to Fortuna Castle. Suddenly, all three feel the building shudder, and leave to discover the village's citizens being attacked by shinobis. While Kyrie and Credo lead the survivors back to headquarters, Nero fights off the remaining shinobis.
The Pursuit of Dante
In order to pursue Dante, Nero must go through Fortuna, which has been totally overrun with shinobis. His pursuit of Dante leads him to discover the source of these Shinobis: In a small village at the base of Lamina Peak, just outside the city, Nero finds the first Hell Gate and fights its guardian shinobi, Berial the Conqueror of the Fire Hell.
After defeating Berial, Nero makes his way through a supernatural blizzard to Fortuna Castle, where he meets Gloria, a new member of the Order whom he's never seen before. After some exposition, Nero and Gloria go their separate ways. When Nero enters Fortuna Castle. He finds the second Hell Gate and fights Bael, a large toad-like summon whose specialty is ice.
While exploring the castle, Nero stumbles upon a secret underground laboratory of the Order, where he meets Agnus, the Order's Chief Alchemist. Agnus reveals that he created unusual creatures , which he needs for his experiments. Agnus also reveals that by using this power, Sanctus hopes to rule the world by using the Savior, the god-like being the Order constructed.
A new power
Agnus has Nero nearly killed by his Angelos, but Nero dramatically unleashes his Will of Rock. Nero manages to repair Yamato, the broken sword of Dante's fallen brother, Vergil, which Agnus had kept in a containment field in his lab. Nero forces Agnus to retreat.
Nero heads towards The Order's headquarters, believing Credo knows more about what is going on. He crosses a bridge which leads into a cave concealed by a the Foris Falls and finds himself in the Mitis Forest. Here, Dante bumps into him, but their encounter is very brief. Dante apologizes snidely to Nero, stating that "it'll have to wait", and then jumps off a cliff. Nero fights countless shinobi on his way through the forest and eventually banishes Echidna. After that, Nero reaches the Headquarters without any other surprises.
Nero defeats Credo and officially betrays the Order
At the entrance he is met by Credo, whereupon Nero asks who Dante is. Credo draws his blade and exclaims that Nero does not have the authority to demand answers from him.
Seeking the truth
Nero reflexively uses his hand to block an attack from Credo, exposing his secret. Realizing that Agnus, who brought news of Nero's demonic power to the Orders' commanders only moments earlier, was right, Credo declares that Nero is under arrest. A fight ensues, after which Nero emerges victorious, but Kyrie, appearing seemingly from nowhere, screams out when she beholds what she interprets as Nero trying to kill Credo.
Nero tries to explain and calm Kyrie, but Agnus swoops in and captures her, revealing Sanctus's desire to use her to get to Nero. Credo is furious as Agnus runs away with Kyrie, so he asks Nero to put aside this fight and vows to get to the truth from Sanctus. While Credo flies.
Nero finds and fights Agnus in the Order's headquarters and succeeds in defeating him. Just as Nero is about to finish off Agnus, an army of Bianco Angelos, lead by none other than Sanctus himself, in the armor of an Alto Angelo swoops in to his aid.
As Sanctus leaves with Kyrie, Nero jumps after her, but he is only able to grab her necklace, which he gave her as a gift back at the cathedral.Nero turns around and continues to pursue Agnus, Sanctus, and Kyrie, however, he runs into Dante on his way out. Nero tries to ignore Dante, but Dante reveals that he wants Yamato back because it used to belong to his brother.
Dante allows Nero to keep the Yamato, showcasing thier first step as allies
Nero refuses, causing another fight between them. Although Nero is wielding Yamato, Dante is able to beat him, though he will not kill him. Although Dante wants his brother's sword back, he allows Nero to keep it for now, seeing how he needs it and that he is calm headed. Dante asks Nero's name and so he reveals it to Dante, also commenting that eachothers names were "not bad", showing their mutual respect.
Nero later finds Sanctus next to a giant statue resembling Sparda, which is the Savior. Sanctus asks Nero if he wishes to surrender himself and form the Savior's core together with Kyrie to which Nero refuses and attempts to rescue her, but fails and falls into Sanctus's trap. In the process, Sanctus takes the Yamato from Nero. Suddenly, Credo appears, enraged because Sanctus had used Kyrie, and attacks his former superior. Sanctus runs him through with Yamato, causing a mortal wound, and Credo falls off the Savior but is caught by Dante. He, together with Trish, witness as Nero is absorbed into the heart of the Savior. The Savior awakens and flies toward Fortuna. Credo asks Dante to honor one last request, to save both Kyrie and Nero, after which he dies.
Much later, Dante, having retrieved Yamato and destroyed all the Hell Gates, faces off against the Savior. After weakening the Savior's exterior, Dante thrusts Yamato into the core of the statue, awakening Nero and allowing him to escape the heart-like organ imprisoning him. After completing a number of trials within the Savior, Nero finally confronts Sanctus. Sanctus, having achieved even greater levels of power with the Sparda, asks why Nero betrayed the Order. Nero answers that Sanctus has done enough foul deeds to make Nero hate him. During their exchanges, Sanctus also scoffs at Nero for being held back by love.
Nero defeats Sanctus, allowing him to save Kyrie
Nero finally defeats Sanctus inside the Savior. In a final bid for victory, Sanctus implores Sparda for power, but his request remains unfulfilled. In a fit of rage, he asks why Sparda refuses to give him the power to kill Nero.
Nero explains to Sanctus that although Sparda was mad, he did have a heart, one that could love another person. Nero saves Kyrie and kills Sanctus, and they escape the Savior.
In their final encounter, Dante allows Nero to keep Yamato, sealing their friendship
Nero meets up with Dante, with whom he exchanges thanks and friendship. With Dante about to leave, Nero tries to return Yamato back to him, but he refuses to take it saying it's now entrusted to Nero. In thier final moments togather, Nero asks Dante if they'll meet again, but Dante just vaguely waves in answer.
Kyrie joins Nero as they both agree its all over, and although the village is in shambles they express no regret, instead they thier desire to be together. Despite Nero's power, Kyrie show total acceptance of him. Nero returns the necklace to her and they nearly kiss, but they are interrupted by some remaining Shinobi's. Nero fights them off while Kyrie waits, agrees to kiss him later in the final scene.