Zachary is Miranda's guardian angel. That's a thing in Cynthia Leitich Smith's Tantalize universe, in which this novel takes place. It's the second in a series, which I read first. Oops.
Anyway, Zachary is having some boundary issues with Miranda that puts them both in a bad position. Unless you like fame, fortune, power, and eternal youth. Zachary is kind of a creeper, to be honest, a little more Edward than is strictly healthy, but at least he's hot.