Nekoda is your typical manic pixie dream girl. She appears out of nowhere. Flirts with our protagonist. Volunteers at the hospital. Says gentle self-deprecating things like, "I babble when I'm nervous" (1.94). And she knows how to wield a sword and will totally kill Nick if she has to.
Okay. So she's not that typical.
Like Caleb, Kody is very good at pretending to be a normal high-school student even though she's actually some sort of demon-slayer whose job is to kill Nick if he turns evil. Just ask Buffy: this is never as simple as it seems.
Kody initially gets close to Nick out of duty, but she soon finds herself actually liking him. She assures her supernatural boss, "If we can't turn him, I will kill him" (3.99). But we have to wonder if she'd actually go through with it.
Kody is another character who, like Acheron, seems really important and then immediately disappears for half the book. She reappears at the end to lend Nick some much-needed zombie-fighting muscle. She also lets Nick ask her out, and she says yes.
That could get awkward, because it's rude to kill on the first date. But if you want to find out about that first date, you'll have to read the next book in the series.