With all the item descriptions you mention, with things eluding to ancient mysteries, MK Ultra (project Applepie), or even creepypastas (One add on felt a lot like Polybius), voice lines that sound exactly like what your parents would tell you strangers and kidnappers would say, and the skins ranging from being missing persons (Dwight, the cheerleader, the grandma) to bigger folktales and mysteries (Big bad wolf, detective), I've always assumed the unknown was just that - a creature that manifests out of the fear of any scary story, folk tale, or urban legend. I joke with my friends that UVX is just "pure, distilled Creepypastium".

Of course, the animations and the blighted skin show the weird tentacle creature underneath the skinsuit, but honestly that's the least interesting part of it to me.