For starters, we need to be clear on what an antagonist is. In any story, the antagonist is the character or group of characters working against the leading character. They are the source of conflict, and in laymen's terms, the antagonist is the "bad guy."

I would argue that the entire Tlic race is the antagonist in this fascinating story. These creatures, reminiscent of centipedes, are parasitic and require human (Terran) hosts for their eggs in order to procreate. A macabre agreement has been reached between Tlic and Terrans that in exchange for the privilege of protection in the Preserve, every Terran family must offer one male to be an egg host. In this way, the Tlic and Terrans have developed a symbiotic relationship of mutualism, in which both obtain a benefit from the relationship.

Despite the benefits that Terrans enjoy as a result of their relationship with the Tlic, the process they must go through to help the Tlic reproduce is brutal. This is demonstrated through the character of Bram Lomas, who arrives at our protagonist's home in great pain, as the Tlic eggs with which he has been impregnated are ready to hatch. We learn that if the eggs are not removed in time, the newborn Tlic will eat their way out of their Terran host.

While the establishment of a relationship with the Tlic was essential for the Terrans in order for them to be protected, it has come at great cost and great pain, making the Tlic the antagonists in this story.