Tatsumi Sekiguchi is a novelist but, as his short stories don't allow him to live properly, he often works for the press. He was in the same college as Kyougokudou and hangs around with him. He's often teased by his friends in regards to his poor social skills.

During the war, Sekiguchi fought with Kiba in the same regiment. While the details about this period are unknown, it is implied that Sekiguchi is a deserter who suffers from psychological disorders because of what he saw and did during the war.

He has a wife named Yukie.