The Yomei Alliance (陽銘連合会, Yōmei Rengō-kai) is the third largest criminal organization in Japan. They are headquartered in Onomichi, Hiroshima. The Yomei Alliance was founded by Heizo Iwami, also the founder of Iwami Shipbuilding. After World War II, he adopted Toru Hirose after he trespassed the shipyard as a boy. He made Hirose an assassin sent to ensure that anyone who could possibly expose the secret of Onomichi to the world was silenced, including even the managers of Iwami Shipbuilding who also acted as executives of the Yomei Alliance and were involved with The Yamato Mark II project. (fathers of Tsuyoshi Nagumo and Takaaki Matsunaga were one of them).
In his later age, Hirose would become the leader of the Hirose Family, which was made up of the children of the men Hirose killed in service to Iwami. Despite Hirose's secret loyalty to Iwami, the Hirose Family was a subsidiary family to the Masuzoe Family, whose patriarch was secretly loyal to the Saio Triad. At some point, Kanji Koshimizu was recruited by Iwami; he became the Captain and second-in-command of the Yomei Alliance, and the patriarch of his own family, the Koshimizu Family.