Junichirō Koizumi serves as the 89th Prime Minister of Japan in this narrative. His background reveals a generational mastery of mahjong, inherited from his grandfather, the "Tattoo Minister" Matajirō Koizumi. This family legacy anchors his expertise and approach to international conflicts.

He stands at the center of diplomatic confrontations where world leaders resolve disputes through high-stakes mahjong matches instead of warfare or traditional negotiations. His role demands defending Japanese sovereignty and strategic interests against global superpowers, a duty he maintains even after retiring from political office. His actions demonstrate strategic prowess and national representation in these encounters.

His formidable mahjong abilities include specialized techniques: "Goumoupai," where he scrapes tiles with a forceful grip to manipulate outcomes, and the "Rising Sun," a strategy centered on the Thirteen Orphans hand configuration. These skills are vital tools in his diplomatic arsenal. The narrative presents him as a master tactician whose gameplay directly shapes geopolitical results.

Koizumi engages in matches against numerous global leaders during diplomatic missions across the world, showcasing tactical acumen and adaptability. These encounters frame international relations as competitive mahjong, where each match bears significant consequences for national prestige and policy. His participation extends beyond his tenure as prime minister, highlighting his enduring influence.

The character's design and thematic significance have been adapted into merchandise, such as card sleeves featuring his likeness, confirming his prominence within the story's universe. This solidifies his status as an iconic figure whose mahjong prowess symbolizes a unique fusion of political strategy and cultural representation.