​How would you write a Fallen Angel?

Hi, I’m new here, name’s Robyn, I looked to reddit to see if there was some group to help me with creative writing and it turns out there is so first of all thank you all for existing here and I bet your all wonderful people! I was wondering what a good way to write a character description would be for a fallen angel? I have the general jist of what I want her to be like but I’m trying to make a list of characters in this story I’m writing and I want this character to be interesting but I’ve never wrote anything about fallen angels or angels in general. The best way to describe her is the main characters best friend, not exactly another main character but not exactly a side character, she has a large spear and half of her body is black, the other side white, she has a short horn sprouting from her head which holds the halo that once sat upon her head, but I don’t exactly know a good way to write how she came to be or how she lost her right to the afterlife, would anyone know a good way to make her make sense?