Eiichi Yamagishi recurs across multiple works by Gosho Aoyama, consistently portrayed as the manager of idol Yoko Okino. His personality is defined by a quiet, unobtrusive nature, preferring to stay in the background. He demonstrates protectiveness towards Okino, including actions like attempting to remove crime scene evidence out of concern for her potential involvement and expressing reluctance to involve authorities upon discovering a corpse.

Yamagishi debuted in the *Detective Conan* series during the Bloody Idol Case as a suspect managing Okino. He reappeared in the anime episode "On Location, TV Drama Murder Case" (Episode 21), managing Okino during a film shoot at Beika Shrine. He also features in several *Detective Conan* manga omakes. In the *Gosho Aoyama Murder Case*, he appears in scenarios where Aoyama is found dead, once suggesting letting the deceased "sleep for another 30 minutes" to play games; in a baseball-themed segment, he participates as "Detective Puppet Yama-chan". In the *Catch the Dangerous Criminal Gosho Aoyama!* omake, he discovers "miraculous glasses" enabling communication with inanimate objects and later joins others reprimanding Aoyama.

Outside *Detective Conan*, Yamagishi makes cameos. In *Magic Kaito* manga omakes, he appears as a cameraman during "filming" and is depicted younger alongside a Gosho Aoyama-like character, participating in setting remodeling. In *Yaiba* Chapter 27, he, Gosho Aoyama, and Yutaka Tani enter a restaurant trap set by Kumo-Otoko and are chased away. In *3rd Base 4th*, he is a member of the Kohnan High School baseball team. In the spin-off *Zero's Tea Time*, he reprises his role as Okino's manager during the "Hanzo" chapter and its anime adaptation (Episode 4).

The character is based on Eiichi Yamagishi, Gosho Aoyama's real-life assistant responsible for designing architecture, vehicles, and weapons, explaining the character's recurring presence and varied background roles.