This is an interesting challenge. On the one hand, your character's unusual gender is a hook that makes them interesting. On the other hand, your chosen pronoun is something that might make an agent need to read your hook twice, which is always a worrying notion.
I'm in favour of Dr. Deb's solution, which is to sidestep the issue entirely by using Loren's name in the hook twice. Something like this:
Seventeen-year-old Loren Brooks fits somewhere between male and female. Loren's new friend Stephen fits somewhere between human and vampire. Theirs a serendipitous friendship, built on the edges of society, and it's going to get them both into a lot of trouble.
In the end, I think it's more important for the query hook to highlight the main character than to highlight a pronoun; I believe having an intersex protagonist is still unusual, so just mentioning that Loren isn't in the traditional male/female binary should be enough to get the agent's attention, and you don't want anything to muddy the waters around that.
As someone who's relatively fond of formal experimentation, I think it's important for us writers to remember that literary agents spend a significant amount of their time reading query letters written by, not to put too fine a point on it, crazy people. And these crazy people may well use terms like "his" "her" and "their" seemingly at random. (I once read of a literary agent who received a query letter in crayon). So it's very important to get the agent to read as much of your query letter as possible before you start doing anything formally unusual (like, say, using a non-standard pronoun), because you want to reassure them that you're a sane person who has thought carefully about your word choice, and knows what they're doing.