I think the angel who was singing is Beel. And the girl is human who also love singing. They met and sang together. Built trust and close friendship. They promised each other that one day they will sing together in a stage and make lots of people who heard their singing become happy.
Something happened which caused the girl’s death. Before death, girl told Beel that they should keep their promise and make that dream come true.
Now, I’m not so sure if the timeline is right, but based on my opinion, that time when Beel as an angel was caught by the armies of heaven is probably after the girl has passed away...
Beel tried hard to finally stand on a stage in the heaven to sing song, even if it was just himself, he is trying to accomplish the dream/promise.
But, before he was able to complete the singing the song, the armies came and captured him.
I think there was a mastermind who is trying to bring down Beel by blaming a crime on Beel.
Beel was captured, sentenced to a punishment of banishment to the hell (imagine as got demotion in a company) and got cursed.
Still not clear if the relationship between Beel and the girl is just love or something deeper than that.
I'm pretty sure that girl is Raphael lol
He loved an angel..?(▰˘◡˘▰)