Sergeant Nero was the seed of an angel and a demon. His angel mother had defied the rules and was punished by LanceSennin. She was sent to Earth fatally wounded. His father came to her, but it was too late. She managed to write a note for him before dying. That his father, Vergil, must train Nero in order for him to not be like his mother. Because LanceSennin said that if Maya, the angel made her son to be a traitor, the god will kill every relative she has.
Vergil shapeshifts into a human and trains Nero to be a proficient and excellent fighter. When Nero was 7 days old, he went to a military school. He graduated with the rank, “Sergeant”. He now fights for good and he will do everything to make Vergil and his god proud. Unfortunately, he never knew about his mother. His only memory of her was a holy necklace that has the ability to make him recover quickly from any injury.
After fighting Alpacalypse at the age of 10 days old, he was put to sleep by a sleeper hold. He continued to sleep for thousands of years. He finally woke when Vergil saved him. Having woke up, Nero is revived and ready to fight again.