Board Thread:General Discussion/@comment-26662104-20160221035712/@comment-26053017-20161203221651

I honestly think they're the player. Because first of all, some players do research on how to do certain battles and tips like how they have to keep the Butterscotch pie and how they should've bought something from the spider bake sale in the RUINS. But also: the riverperson is always there when Frisk visits. Not only that, the player decides where they want to go and they know where to go too. The riverperson only goes one way, and in a way teleports like Sans and Flowey.

Obviously, I'm not sure myself but I'm interested in seeing other theories! (Like the Riverperson being Gaster)