Angels shouldn't pal around with demons, especially not ones they've been sent to assassinate. They shouldn't work with them or cook them dinner, or strike a deal with the Devil for them either. Sunshine probably isn't a very good angel.
The demon in question is Felix Specter. He's forgiven Sunshine for stabbing him all those years ago. He's got a lot on his plate: a curse, his mental health, and a questionable love life. Finding out Sunshine owes the Devil a few favors doesn't help. Neither did that kiss, not that Felix will talk about it anyway.
On top of everything else, Sunshine and Specter have a case to solve. They handle issues that human detectives can't, like monsters in the sewer, mages doing illegal charms, and runaway witches. But they've always dealt with anything the city has to offer. What could be different this time?