[Note: Even though the plot seems pretty far from primary when experiencing this head-trip of a book, the following contains details that could be considered “spoilers.”]
The Box Man starts and ends with a kappa, and we see everything it does. The creature hops on the back of a passing scooter, bringing us along as it observes and becomes part of an incredibly strange adventure.
The kappa is “easily the single most famous yokai in Japan”, according to Yokai Attack: The Japanese Monster Survival Guide. Among its other, nastier traits, the child-sized water-monster has a beak-like mouth and a shell like a turtle, and it likes to cause trouble.