It would be easy to dismiss Jamie as pure comic relief in The Demon's Lexicon, especially as he delivers lines like, "My life was going to flash before my eyes, but it decided to hide behind my eyes and quake with terror instead" (1.96). But Jamie does far more here than just provide laughs.
For one thing, he's an openly gay character who is written as an average teen. He's not flamboyant (unless you want to make something of his lavender shirt or his earring), he's not oversexed, and he doesn't lisp. In other words, he's portrayed as a real person, not a stereotype. And he only gets realer as he provides an interesting contrast to Nick.
Both Nick and Jamie are sixteen-year-old boys, but beyond that, they don't have much in common. While Nick is hyper-masculine and aggressive, Jamie is sensitive, nervous, and self-deprecating. Nick has trouble expressing or understanding emotions, while Jamie seems to be a student of human psychology. Note: that's human psychology, which could be why he has a hard time understanding Nick (13.53).
And finally, while Nick can't seem to figure out who he is, what he feels, or where he fits, Jamie—like Mae—has a pretty good understanding of himself. When they go to the Goblin Market, Alan remarks on Jamie's dress, saying, "I see you're not dazzling us all like Mae," to which Jamie responds, "Well, I thought—I thought that I usually look like what I really am" (4.120-121). Ain't no shame in the being yourself game, Jamie.