A mysterious wandering man with a vast knowledge of the supernatural and is mystical in appearance. He carries around a large wooden box filled with medicine and holy tools. Among them is an enchanted sword that is used to slay the mononoke. To use the sword, the Medicine Seller must find the "Katachi" (shape), "Makoto" (truth), and "Kotowari" (reasoning) of the mononoke he wishes to exorcise. Once the sword has been unsheathed, he transforms into a demon-like creature. While his origins are unknown, it is clear that the Medicine Seller is not human or at least not entirely human; he has elf-like ears, and pointy canine teeth. Even in the time-skip Bakeneko arc, the Medicine Seller still appears as the same handsome youth from the previous arcs (the only differences in his appearance from previous arcs is he now wears small round red stud earrings and a ring on his index finger).

Spoiler

It was revealed in the Umibozu episode when the ayakashi asked the passenger's fears, the Medicine Seller calmly answers he is afraid that there is no form, truth, or reason at the edge of the world, thus no longer having a reason to exist.

(Source: Wikipedia)