With Nero as the protagonist, he's gonna need a team of his own joining him in the frontlines. Nico just stays in the van as maintenance.
For future antagonists, how about twin daughters of one of Mundus' generals? Right-hand of Mundus was once Sparda and left-hand is another demon who's jealous of Sparda's strength. Eventually, he/she takes advantage of this, when hearing of Sparda's betrayal and giving birth to half-breed twins. Taking this as inspiration, he/she seeks out a human mate of his/her of her own, the same way. Only for the side of evil.
Sparda (named for spartan & Spada - it. for sword). How about Lancia (it. for spear)? Twin daughters where one wields an axe and the other a spear. For their goal, to defeat Sparda's children and those who fight beside them. And help wipe out humanity with any powerful demon who seeks to take the throne.