so, think of your background like a timeline and work backwards: 
PRESENT: 
- who are you now? are you happy this way? do you have unfulfilled wishes and desires? what DO you have that you're grateful for? is there something you're taking advantage of?
- this can refer to people, places, things, emotions, relationships, powers, tangible and intangible things. this will probably be the shortest part of your bio if you want to open it up to the most people. i leave my bio's open ended so usually the "present" is vague, but i think about these questions a lot esp when coming up with plots!!

PAST 
angel: 
- was your fall intentional or not? Do you remember falling? do you have residual powers from back in heaven? how did you find out about your powers? how did you find out you were on earth? reactions to leaving heaven? how's life been in the span between falling and present? fill in the blanks!!
human: 
- what caused you to wish for what you are wishing for? is there one singular reason? is there a series of reasons? how important is that thing to you and does it change when you meet an angel? how integral is this wish to your every day life? how much does it REALLY mean to the human - like its one thing to consciously want something and then actually getting it, there might not be the same feelings. 
- is this a wish that is or isn't humanly possible to attain

PAST (FURTHER BACK)
angel:
- who were you then? is it the opposite of who you are now or does it lead to who you are now? is there some fond memory you have? 
- since you're not a demon yet, you have some good still left in you, so you probably still have friends or good memories. also, in heaven, im sure you were at your peak, so what is your characters BEST self. flesh it out and plot friendships or frenemies with people! also find out what fundamentally drove your character to fall - it couldn't have been without reason, even if he doesnt remember it.

human: 
- If you're a human think about past lives maybe? or your parents, like what influence do they have on your char
- what was the environment as a child? what about friends, pets, cousins, extended family? past relationships? traumatic or uplifting experiences? what is the seed planted at the beginning of your story.