This feels so clear that it wouldn't surprise me if it's confirmed in one of the secret scenes and I just didn't see it. If Aspen is not a vampire, then I have been thoroughly and convincingly bamboozled.
- he doesn't have windows in his house (light sensitivity)
- ditto for mirrors (a vampire wouldn't have a reflection)
- in the scene where he checks your knee in the cabin, he spots a tiny bit of blood and seems very flustered about it
- during the party, you can ask him what his favorite drink is, and he says Jose doesn't carry it ("I do not drink... wine.")
- also during the party scene, there's a line which mentions that he doesn't fully show his teeth when he smiles
- ALSO during the party, his body is cool to the touch when you dance with him
- subtle design detail, but he wears that choker thingy that covers a bit of his neck. Hiding a scar, perhaps?
- less certain about this one, but he does ask permission before entering your room. I think the same is true of Ines, but it could be significant
Now granted, there are a couple problems with my theory, which could be chalked up to variations on vampire lore:
- obviously, Aspen walks around in daylight all the damn time. However, there is plenty of fictional precedent for vampires that can survive in daylight (such as the Twilight series, or even the original Dracula novel), or he might be a dhampir/daywalker. (There are also some American folklore creatures which are like vampires in a lot of ways, but also a little different. That could fit with the old west setting, and be a nice secondary twist after all the "definitely vampire" hints). If he *is* a vampire capable of being out in daylight, it's even possible that he came to Sunshine's Peak BECAUSE it's daylight so often--maybe all the sunshine helps him keep control? That could also explain why he reacts more sharply to seeing your blood when inside his cabin, where there's no sunlight.
- something that puzzles me a *little* more is that he appears in Moxie's photos, which if we're going by the logic that he doesn't have a reflection, you'd think he wouldn't appear in photography either? But then again, those are seemingly magic photographs, which show things that haven't happened yet(? or maybe things that happened and then got erased? or maybe things that Ash dreamed?) Moxie's photos are weird and unexplained in their own way and warrant a separate theory.
If this theory is true, it would also lead me to believe that the second nightmare is about Aspen. Granted that scene feels more werewolf than vampire, but there is the neck biting and sexy undertones (overtones?). The creature in that dream also behaves much more human than the monster in the third dream, so I don't think they're the same. But if Aspen is a different kind of blood-drinking creature, maybe he will eventually turn into a big monster? But I think they're unrelated, and I also don't believe Aspen is responsible for the murders... like I said, the hints that he's a vampire are pretty overt, so it would surprise me if it was that straightforward.
Cue the comments of "he actually straight up tells you if you do X, Y and Z, you nerd."