Wanyuudou is a supernatural entity and the first companion to Ai Enma. His existence began over 400 years ago as the left wheel of a royal carriage. During a wartime escape, the carriage carrying a princess and attendants was struck by fire arrows, plunging off a cliff and killing all aboard. The burning left wheel gained sentience, transforming into a tsukumogami that manifested as a flaming wheel with a human face on its hubcap. This form haunted humans near the cliff nightly until Ai Enma offered him a place at her side and granted him a permanent human form.

In his human guise, Wanyuudou appears as an elderly man with a bald head, narrowly opened eyes revealing black irises when fully open, and prominent black eyebrows. He traditionally wears a dark reddish-brown haori over a khaki yukata, secured by a black obi, with a white nagajuban underneath, black tabi socks, straw zōri footwear, a light gray fedora, and frequently a red scarf. His attire evolved in "Three Vessels" to a brown haori, dark gray yukata, and purple nagajuban, with the red scarf rarely worn. Upon his initial transformation centuries prior, he wore white garments with a black sweater and a large straw hat.

Wanyuudou exhibits a calm, mild-mannered, and generally non-aggressive demeanor, maintaining composure in tense situations. He possesses extensive knowledge of supernatural phenomena, often deducing events before his companions. Obedient to Ai's commands, he occasionally displays humor and a habitual smile, and is a habitual smoker. While loyal and affectionate towards Ai and her other companions, he harbors consistent dislike for Kikuri.

His multifaceted supernatural abilities include traversing between the human world and the Realm of Eternal Twilight. He magically alters his attire for investigations and manipulates human perception to implant false memories or identities. At Ai's request, he transforms into a black straw doll adorned with a red string around its neck, often preceded by adjusting his scarf or tilting his hat. His original youkai form is a sentient, flaming wooden wheel capable of levitation, flight, and igniting nearby objects. He can further manifest as the complete fiery black-and-red carriage he was once part of, bearing the black hell-mark symbol; this levitating or flying carriage serves as Ai's primary transport for soul retrieval and offers some magical defense. He possesses any wheeled vehicle or turbine, controlling individual wheels or the entire apparatus. Despite his aged appearance, he demonstrates formidable martial prowess and superhuman strength, evidenced by stopping a falling truck. Fire manipulation is a core ability: he perpetually burns harmlessly in wheel form, ignites objects nearby, and precisely hurls fire projectiles in human form.

Following Ai's self-sacrifice and death at the end of "Two Mirrors," Wanyuudou initially lived peacefully as a human construction worker. Kikuri later recruited him back by announcing Ai's return. Throughout "Three Vessels," he operated undercover as a janitor at Yuzuki Mikage's school in Saigawara City, aiding the investigation of clients. Near the season's conclusion, when Yuzuki temporarily assumed the role of Hell Girl, Wanyuudou was the sole original companion to offer her direct assistance. He ultimately resumed his duties under Ai after she reclaimed the Hell Girl position. His name translates to "wheel entering the road" or "wheel monk," referencing his origins and appearance.