The core conflict usually starts with a fundamental opposition of nature, which is way more interesting than just good versus evil on a cosmic scale. It's about internalizing that cosmic war. The angel character isn't just pure; they're often bound by rigid doctrine, celestial law, or a duty to judge. Falling for a devil forces them to question the very foundation of their identity and purpose. Is their love a corruption, as their kin would say, or is it a higher form of understanding? They have to grapple with the fear of falling, not just in love but literally—losing their grace, their home, their entire sense of self.
From the devil's side, the conflict is often about redemption versus damnation. Many stories play with the idea that the love of an angel could 'save' them, but the more nuanced tales explore how that's its own kind of condescension. The real tension is whether the devil even wants to be 'redeemed' into a system that cast them out. Their love might be the one pure thing in their existence, making them vulnerable and perhaps even worthy of destruction in the eyes of their own infernal peers. The fear isn't of falling, but of rising into something that feels alien. It creates this beautiful, tragic push-pull where being together means betraying everything they are, while being apart betrays their hearts.
I've always been drawn to stories that let the angel get a little morally grey and the devil show unexpected honor, because that middle ground is where the real relationship has to live.