Everything Is Wonderful Now - The Requiem Series, #1

When Dark is good and Light is evil... Ten-year-old Sera is isolated and depressed; bullied at school and at home by her father. Her mind gives her no respite, suffering from a panic disorder and a second voice in her head that forces violent, intrusive thoughts. She meets the fallen angel Byleth one night while wanting to end her life. He isn't a fan of kids and intends to leave immediately, but Sera's aura catches his eye.
It's dark, and it isn't from her trauma that pulls at his heartstrings. It's also attracted a corrupt angel who wants to rid the world of another Hellspawn child. Sera has a lot on her plate, but as she ages and transitions to Sean, a disabled trans man with C-PTSD, he not only meets more spirit guides from Hell, he realizes that his biggest threat has always been the 'other' that dwells in his own head.