Where’s the line between fictionkin and otherkin? For instance, if we define otherkin as “non-human soul” and fictionkin as “soul of a fictional character”, there’s still a ton of overlap. Dragons and elves are fictional, but elf/dragonkin are otherkin. Homestuck trolls are non-human, but they’re fictionkin. Yet, both of these could qualify for either label. How do you differentiate the two?

Its a very fuzzy line indeed, and someone can be both otherkin and fictionkin at the same time. For instance anyone who has a fictotype that’s a nonhuman character is also otherkin.

As for the line where otherkin become fictionkin: if you can point to a specific author that created the race that an otherkin is, that is fictionkin. If they’re a race that’s common in folklore/whose original author has vanished in the mists of time, we just consider that otherkin.