Yoichi was a student at Keio University, and when his family's rural dyestuffs business failed, he decided to pursue his interest in programming.

In 1982, the company released the erotic title (eroge) Seduction of the Condominium Wife (団地妻の誘惑, Danchi Tsuma no Yūwaku), which was an early role-playing adventure game with color graphics,[2][3] owing to the eight-color palette of the PC-8001 computer.

The game went on to receive numerous awards, and Koei produced several more such games set against the backdrop of world history, including Romance of the Three Kingdoms, set during the Three Kingdoms period of Chinese history, and Uncharted Waters (大航海時代, Dai Kōkai Jidai, lit.

This subsidiary localized Koei games for export to all territories outside Japan, as well as producing original games and concepts with the leadership of designer Stieg Hedlund, like Liberty or Death, Celtic Tales: Balor of the Evil Eye, Gemfire and Saiyuki: Journey West.

Koei's Ruby Party division specializes on games labeled as Neoromance: GxB dating sims, usually with extra side-quests.

Harukanaru Toki no Naka de is a newer Neoromance hit, with many sequels and an anime television series based on it.

[13][14] Koei's subsidiaries in the United States, Europe and Korea already had their names changed months before the Japanese parent.