Yamaguchi, formally titled the Shop Manager, oversees daily operations at Books Takiwa, a bookstore employing Jun Sakurada and Saitou. His full name remains concealed through most of the Unwound World arc, revealed solely by the Head Manager in its final chapter. Standing 170 cm with brown hair and blue eyes, his professional demeanor aligns with his managerial duties.

Interactions with Jun and Saitou underscore his balanced approach as a supportive yet pragmatic authority. A defining moment occurs when he overhears discussions about Jun’s potential promotion, leading him to privately apologize for overburdening the latter. This vulnerable display of humility resonated deeply within fan communities, cementing the scene’s emotional impact.

Though central to workplace dynamics, his background receives minimal elaboration. His surname—derived from the Japanese characters for "mountain" (山) and "entrance" (口)—hints at themes of stability or transition. Official sources omit details on his personal history, external relationships, or involvement in overarching conflicts like the Alice Game.

His narrative remains confined to professional exchanges with Jun and Saitou, detached from the series’ supernatural elements. As a secondary figure in the Unwound World timeline, his motivations and growth are subtly inferred through workplace interactions, with no explicit expansion beyond this role.