Ryuji is seen accompanying John Bacchus whenever he is in his office. Ryuji assists John in connecting Kamado Ueshita's Server Diary to the HOLON III network, giving every citizen in Sakurami City an Apprentice Diary.
When Minene Uryu, Masumi Nishijima, Hinata Hino, Mao Nonosaka and Ouji Kosaka infiltrate the Quad Towers where John and his two HOLON III terminals are located, John attempts to predict their Diaries with his own "The Watcher" diary. However, the five of them trick John's diary by writing in their own false entries, eluding John's security forces that were sent to capture them.
Ryuji proves himself to be handy at this moment. By using his "Secretary Diary", he can predict John's future actions regardless of the false entries. With this method, John eventually locates and traps Hinata, Mao and Kosaka, while other security forces ambush and kill Nishijima, with Minene losing a hand due to a claymore planted in advance.
Ryuji accompanies John into the Gasai Safe, a high-security safe in the building. He is later shot by John for unwittingly giving away the keycode to the vault they were locked in, after Yukiteru Amano tricks his own diary to reveal his future capture and interrogation of Ryuji, which allowed Yuki to gain the keycode for the first door of the safe. In the anime, he is shot by Minene and dies from blood loss. He is seen alive again in the third world.
- His surname contains 2 Kanji: Kuro (黒), saki (崎). It means "black mountain route".
- His given name contains 2 Kanji: Ryu (竜), ji (司). It means "the place of the dragon".