Yamaguchi Ryugen

Yamaguchi is Ookura Kaito's (former) superior, war buddy, and lover. They turned to drugs together after a particularly traumatic event in the Iraq war in 2005, and continue to do drugs while Ookura is in prison. This eventually costs Yamaguchi his job. Despite no longer serving in the self defense force, their reputation as battle legends persists, earning them the trust of serving policemen who recognize their faces.

Ookura and Yamaguchi are sincerely in love with each other. The two met many years ago in Hokkaido, at a small pond. They went through the trials of war together and endured many hardships at each others' sides. Even while Ookura served his prison sentence, Yamaguchi worked at the prison in question and continued to stay by Ookura's side. Their relationship was a secret from everyone.

Initially, Ookura is hesitant to kill world devils due to his attachment to his reserve tank. Yamaguchi is furious with him for this upon finding out, and reminds him of his duty as a soldier to place the world before the people he loves. While he does not understand the battle Ookura is fighting, he does understand the horrors and necessities of war. He is last seen shooting up in a park, happy knowing he is about to die so that Ookura can help save the world.

According to his description, Yamaguchi is twice Ookura's age.