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We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
In a time where there are two subspecies of humanity—Hybrids and non-Hybrids—17-year-old Ada knows she's the girl-in-between. Decent hook, I'm curious what these two types are and how she's not either. She doesn't have the preternatural agility or intellect she's been told the Hybrids have. And while she doesn't consider herself to be that pathetic, pathetic how-so? are they simple regular humans like us? Or are they incapable in some ways? like the non-Hybrids that run her village, she knows she's a long shot from being a member of the elite. You might want to be careful with wording, b/c if you're trying to say "elite" and "Hybrid" are the same thing it could get a tad confusing. The Non-Hybrids can't be completely insipid b/c they run a village. But with her friend Zeran already showing the potential to be classified as a Hybrid, she fights to excel in her upcoming tests for genetic classification. Because becoming one not only means seeing him again—but a chance to meet her Hybrid parents for the first time. Couple of questions raised here - if he is her friend then we assume she sees him on a semi-regular basis already. If Hybrids and non-Hybrids are kept separate after being classified you should rephrase the "seeing him again" line, because as of right now it just sounds like she'll see him again at the tests, which as I said, shouldn't be such a huge deal since we assume they already see each other somewhat regularly. Second question that arises - how does she know her parents are Hybrid? I'd consider working that information into the beginning where she is confused about her origins - like, if she has Hybrid parents, why isn't she smarter / faster / better, etc? And, both of these last sentences imply that the H and non-H's are kept separate but you don't actually say it. You need to be clear on that point.
When a series of mental, physical, and (of course) deadly I think you should rephrase to "potentially" deadly, because if all of them were flat deadly, all participants would be dead. tests prove she is a Hybrid, Ada is thrilled—until she learns her suspicions have been true. She is intermediately unique, having their DNA but lacking the potential to fully sustain herself with no sleep or nourishment, or effortlessly contribute to the harsh intellectual demands of their society. This sentence is a little convoluted at the end, I'd rephrase. When her father casts her off? as worthless to humanity, Ada is distraught. She disappears off I'd curt "off" here into the night with plans to kill herself.
But plans change, I think I need to know why she decides not to kill herself and Ada get lost in the Hollows I wouldn't worry about properly naming the Hollows here, as it only occurs once in the query, simply describe it and kill the dashes for flow—a vast wasteland separating the Hybrid society and the villages—in her pajamas. Thoughts of Zeran drag her from depression, If he is the reason why she decides not to kill herself I think that should be made clear at the beginning of this para and she uses her limited Hybrid abilities to try to survive in a land of man-eaters and deadly dust storms. Fun. I'd kill "fun" it doesn't fit in with the rest of the query voice you have here.
Acquiring the company of a group of non-Hybrids, life becomes possible, but more confusing. Because the group has a goal; make it to the island of refugees alive refugees from what? Are these Hybrid rejects, or just regular Hybrids from her former land?—possibly the only place for her. Ada's problem? It will mean leaving the boy who had always loved her, kill the comma behind forever.
In order for that last bit to pack a punch, we need to know how Z feels about her exile. Did he promise to come for her? Does she believe he's out looking for her? The only thing I know about him right now is his name, I don't know why she would hesitate to leave unless he had made some sort of vow or tried to stop her Exile from happening. Bring some clarity into the query on the points I commented on above, and give us a reason to believe in the relationship. If he's the reason she chose not to kill herself in exile, she must have some hope of them being together - but why? Something he said? Something he did? The query needs more Z in it if this is a star-crossed lover story.