Spoiler: Helios Magical Research
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Possession, surprisingly, has not come up very often in the Suneater Cult up to this point (as far as you are aware, at least). The Suneaters tend to summon distinct beings that form bodies out of magic, rather than spirits that require a host. Based off of your research, the main reason behind this seems to be that a possessed spirit does not automatically gain dominion over a the being it's possessing; it's a battle, often long and drawn-out, and something like spiritual "blending" is not an uncommon end result. In other words, if you summon a spirit to possess someone, in the worst case you might just give that person the powers of that spirit and a reason to hate you.

Going off of this, you're reasonably comfortable in deducing that the person in front of you is possessed, but also that the possession isn't fully "complete" yet. On first brush, they come across as a fully mundane human; no magical aptitude, no special powers, completely standard (though perhaps a bit malnourished). Your deduction that he is possessed comes from the signs that aren't there, as well as what Lily has told you about what she's smelled. Based on what she said, the "pests" you're dealing with fall into three metaphysical categories: predators (snakes, spiders), scavengers (rats, mosquitoes), and parasites (tapeworms, roundworms). A predator spirit would be obvious and possibly territorial; it's more likely that this person is possessed by a scavenger spirit or a parasite spirit, both of which would take a long time to fully "consume" their host... and both of which would have reason and ability to hide from easy detection.