The two, a boy and girl, were fairly young when they met on Earth, years before. Almost, but not yet adolescents.
The young girl was an angel, while the boy was a demon, yet each, at the time, thought the other to be no more than human...and they were content to have the other think the same. Even as they became close friends, neither revealed their true nature, both knowing it was dangerous to tell (all that magic to keep them from being identified as what they really were wasn't for nothing, after all), and fearing their friend's reaction to it.

Along with all their shared misunderstandings and secrets, they had something else in common. They were considered a waste of talent by many of their peers....as despite having great power and potential, the boy had no interest in joining the armies of Hell, just like the girl was set on never getting involved in the war against demons.

Some time passed before the young demon found out about his friend's true nature. He didn't mention it at first, but eventually unable to keep it all secret anymore, he decided to just get it over with and tell her he knew, as well as what he really was himself...even if it probably meant her never wanting to see him again or worse.
Such, however, didn't happen. Unlike he feared she didn't seem to mind any more than he did. Burden of lying to eachother like that lifted, they only became closer.

Getting a bit older they became closer yet, falling in love.

With their longer and more frequent stays on their Earth safe havens to be with each other, even with how careful they were, eventually Heaven found out about them.
The angel, though, told her beloved sincerely she didn't care if she was cast out of Heaven, that it only meant they wouldn't have to hide anymore. So as she was called to answer for her so-called crimes, she left to tell those accusing her just that.
Yet, when she met him again, she was not a fallen.

Refusing to even go near him, she said she now saw things clearly. That what they had would never work, that what they had been doing was wrong...
The demon was confused, particularly seeing she didn't seem to mean it, knowing her well enough... Yet the girl was so firm about it, right to the moment she left him to 'be with her own, where she belonged'...leaving him for good.

With time, his confusion turned to anger, and that then would turn to bitterness. Heartbroken, lost and irate, he decided to turn to his own, just as she had.
He joined the armies of Hell, and in a few years went up in ranks quite quickly.

And it was in service of hell that he would come across her again.
He couldn't find it in him to kill her...and having been aching for answers as to why she left him all this time, well...he couldn't just let her go and let the opportunity slide, now could he?