The Yamato Mark II, better known as the "Secret of Onomichi", was an Imperial Japanese Navy Yamato-class battleship that was laid down at Onomichi and completed in August 1945 during the last days of World War II. The ship was built by Iwami Shipbuilding on a contract from the navy, and Iwami Shipbuilding employed Chinese immigrant labor supplied by the Saio Triad.
However, as the Japanese government began to consider surrendering to the Allies, the contract was cancelled and Heizo Iwami left uncompensated. Admiral Minoru Daidoji, a hardline militarist, shifted misplaced war funds towards the resumption of the ship's construction, and the Yamato Mark II was finished just before the Japanese surrender in September. After the end of the Allied occupation of Japan in 1952, Daidoji entered politics with the conservative LDP and used his "war hero" status to amass several political connections, forming the secret "Daidoji Faction" and controlling Japanese politics as "the Fixer." Taking advantage of this, Iwami blackmailed Daidoji and his faction into bankrolling his company's expansion in exchange for keeping the Yamato Mark II hidden in an underwater drydock. The ship was nicknamed the "Secret of Onomichi", as it was Japan's highest-classified military asset, and its existence was physical proof of collusion between corrupt politicians and Iwami, who had founded the Yomei Alliance yakuza clan with the sole purpose of keeping the secret.
In 1976, by which time most of the men involved in the ship's construction had died, Daidoji asked Iwami to destroy the ship to ensure that the "Secret of Onomichi" never leaked. However, Iwami instead decided to have Toru Hirose kill those who were aware of the secret, leaving only himself, Hirose, and the Saio Triad boss in the know about the secret. When the Saio boss died in the 1980s, he only left clues about the secret to his son, Big Lo. Concerns about the secret reemerged in 2016 when, after Tsuneo Iwami failed to kill Big Lo's son Jimmy Lo, Heizo Iwami grew concerned that Big Lo would attempt to reveal the Secret of Onomichi to cause a political scandal. Ultimately, Big Lo enlisted Kazuma Kiryu and the Hirose-gumi's help with discovering the secret, and Kiryu used the code that Big Lo's father had given him to find Yamano Dodai's grave and the directions to the warship. After Kiryu and the Hirose-gumi fought their way through Yomei guards at an old Iwami shipyard and Naoto Tagashira pulled a lever, the ship emerged from its underwater drydock for all of Onomichi to see, causing Heizo to punish Hirose by killing him. The media frenzy that ensued led to a dying Daidoji abandoning Heizo and supporting his son Tsuneo's ambition to take over the Yomei, and Tsuneo had his father killed and his death passed off as a suicide. Kiryu would go on to defeat Tsuneo at his father's funeral at the Iwami shipyard, and Tsuneo was ultimately arrested for his father's murder.
