Angel's Grace. The Demon's Series, #4

The Demon's Series: Book 4 If Bright had known how good it was to be a reformed demon, it would have reformed millennia ago. Not that reform has been trouble-free for it, granted. Its presence in the demon hunters' guild had been so divisive that Bright had to leave. Now Bright and its friends - Sunrise, Raven, and Mercy - hunt demons as an independent team in a foreign nation. When they stumble upon a strange new kind of demon, asking the guild for assistance means risking another backlash. On a more personal level, Bright knows its three teammates would like to forge a romantic relationship with one another.
Even its teammates know they want a romantic triad. But for some reason, they can't just do that. Humans are incomprehensible. Still, the most worrisome issue is that some demons in Anesh have been working together to stop Bright. They've attacked Sunrise once, and Bright expects them to try again. Or to attack one of its other allies. Its concerns are justified- -but their plan is far worse than anything Bright imagined.