Lili is a Mesopotamian siren, and life as an immortal being is hard enough as it is. She's asexual (which is incredibly difficult to reconcile if your entire point as a mythical being is to seduce people to death). She's also struggling with depression from being alive for so long.
Lili is an absolutely shoddy improv-detective trying to track down a serial killer so ruthless that it makes even her murderous soul uneasy. However, there's something larger at work than just one serial killer. A small town is hiding an even deadlier, global-scale secret. Forget Area 51 conspiracies. This one beats them all. With magic.
So, what better way to spice up her eternal life than being hired as a vigilante detective to stop a serial killer? Anything, literally anything. She'd trade her left lung to get out of this. Or, perhaps, somebody else's.