We've already seen the Demon King
Kyle is somebody high-ranked in Shadow hierarchy. He is the Demon King or will become one by the time Stella reaches Hilla.The overwhelming majority of Shadows (that dont work with Pyre Knights) just do their simple jobs, and it takes up all their time. They never venture outside to study yumans like proto-Rose did. Furthermore, Shadows seem to see themselves as expendable, not worthy the complex rescue operation seen in Rose and the Thorn, which suggests those two are somebody important. This makes Kyle the most likely opponent to Stella when the time comes. Out of the characters we've seen so far.
Moxie once belonged to Debbie
Moxie used to be a capybara. Debbie used to own a capybara named Capybara back when she lived together with Chloe.Fairies are creatures of pure information.
This is hinted by page number 403. This is the number of "Forbidden" HTTP error, and this is also the page, where Svana takes away Bridget's phone and forces her to save Willa. Pokey is related to Li'l Sebastian.
Leslie mentioned he sired over 200 mini-horse children, and if anyone could travel between worlds it would be Li'l Sebastian, perhaps in 2003 when he was out of the public eye due to an "ankle injury". Stella's abilities come from her mother more than her father.
She claims to be the daughter of the dog god Spar-Ao-Kay and believes her Super-Strength and whatever other "demigod" powers she has come from his divine genes. Though what we've seen of Spar-Ao-Kay hasn't suggested he's anything but a big red hound who's believed to be a god. But he did once have an affair with a "fairy" in dog form and Stella does have fairy barcode stripes down her back.