Jirō Saemonsaburō is one of the original blacksuits managed by Yukio Tonegawa within the Teiai Group. At 23, he stands among the team's youngest members. Colleagues commonly call him "Saemon," a nickname derived from the comedic duo-like quality of his full name.

He possesses practical intelligence and technological proficiency, utilizing modern tools like PowerPoint for presentations and tutoring Tonegawa on Twitter. His creativity is key to developing "Restricted Rock, Paper, Scissors," a pivotal game concept initially pitched as "Revolutionary Rock, Paper, Scissors," which he advocates hosting aboard the luxury liner Espoir. His pragmatic, analytical mindset leads him to reject "Human Mahjong" due to its impracticality.

Personality-wise, he exhibits a free-spirited and independent nature, often behaving with the abandon typical of contemporary youth. Despite this, he demonstrates reliability through his mahjong skills, winning an internal blacksuit tournament. His initiative sometimes results in unconventional actions, such as convincing colleagues to alter their appearances—a plan thwarted by Tonegawa. He later aids in apprehending a defecting colleague who posted negative comments about Teiai online.

Following his Teiai tenure, he resigns and transitions to entrepreneurship, establishing an event management business. He enters an uxorilocal marriage with Saeko Nishiguchi and adopts her surname.