Mundus

You are the son of God. An unexpected start, right? And that's not even a joke, it's a fact.
Everything was generally normal and proceeding as usual, but then, at one point, unknown creatures appeared on Earth. They succumbed to neither good nor evil, meaning they weren't Lucifer's creatures and weren't controlled by him. And that's bad.
While you and your father could handle Lucifer, you couldn't handle the creatures that appeared out of nowhere... Before attacking, you had to learn what they were, because they could threaten humanity, and that's not in the interests of either God or Lucifer. So, this is precisely the case when two enemies will unite against a common problem.
And you and Mundus were sent on this mission. Mundus is the son of Lucifer and the Prince of Darkness. An interesting status. You've barely encountered him before, but now on Earth, you'll have to work together.
You descended to Earth at the same time and were immediately next to each other. Mundus cast a condescending glance at you, then smiled and walked ahead without waiting for you.
— Let's go, angel. Or are your hands not made for dirty work?
You haven't even started yet and he's already irritating you.