Shinji Saegusa presides over Hoshimi Production, a small idol agency based in Hoshimi City. His career began at the major entertainment conglomerate Van Production, which employs over 2000 people, before he departed to found his own independent venture. This move branded him a traitor in the eyes of his former colleagues at Van Production. A deep-seated mutual resentment exists between Shinji and Van Production's president, Kyoichi Asakura, fueling a rivalry that extends to their respective idol groups.

Shinji's past includes scouting the rising solo idol Mana Nagase, who later joined Hoshimi Production. At Mana's personal request, her high school classmate Kōhei Makino became her manager within Shinji's company, a request Shinji granted. Following Mana's death, Shinji continued leading Hoshimi Production, guiding its expansion and overseeing the formation of its idol groups, Moon Tempest and Sunny Peace.

Physically, Shinji sports short dark gray hair, shaven at the back, and a matching goatee. His eyes are light gray. He typically dresses in a gray dress jacket layered over a gray long-sleeve sweater, complemented by black dress pants and shoes.

In demeanor, he carries an air of aloofness coupled with an unusually distinctive presence. His driving motivation remains building and sustaining his independent agency in direct competition with his former employer.

The name Shinji Saegusa derives meaning from its kanji: "Shin" (信) signifies "trust" or "believe," while "ji" (司) means "officer" or "boss." His surname "Saegusa" combines "sae" (三) for "three" and "gusa" (枝) for "bough" or "branch."