Kinji Tohyama is a student at Butei High school. 

He was ranked Level E (the lowest combat rank) due to missing the Butei evaluation exam. However, under certain "conditions" his level raises to the highest combat status of Level S (the highest combat rank). He is currently enrolled in the Inquesta department. After the death of his brother, he now plans on quitting Butei and live a normal life by transferring into a normal high school. His duty-issue weapon is a Beretta 92FS Inox (which was given an illegal Machine Pistol conversion and referred to by him as Beretta Kinji Model), and a butterfly knife. In Volume 6 of the light novel he obtains a black Desert Eagle (which was similarly modified to fire in burst as well as auto) which belonged to his father and a double-edged scramasax, (modified with a hidden sheath and a reinforced rubber handle) which belonged to Sherlock Holmes, to add to his arsenal.

He has accepted the mission of clearing Kanzaki Kanae's name by request of Aria. Kinji cares deeply for Aria and eventually develops romantic feelings for her as the story progresses. Before those events he was searching for details concerning his brother's, Tohyama Kinichi, death, but found out through Riko that his brother is alive and is a member of IU. He also admires his brother Tohyama Kinichi deeply, yet fought against him for Aria's sake. He won, and told him to find another way to destroy the IU.