What’s the climate like in Johto? It doesn’t seem to get much rain. If one of the primary building materials up until recently was paper, I would assume that even humidity must be fairly low for that to be viable.

In truth, anonymous, Johto is temperate, and it has a monsoon season noted for its intense humidity and constant rain. Keep in mind, of course, that paper walls are mainly an interior feature (or an interchangeable exterior feature that may be swapped out or covered up with bamboo during the wetter months), so the likelihood of exposing these walls to rain is slim. Meanwhile, humidity is less of a problem for the particular kind of paper used in Johtonian architecture. The paper itself is a bit more durable than the sort you’d find in your printer.