Kozaru (spelled Koen in Yen On's translation) is a Human Assassin-Tracker from D.D.D. He makes a brief appearance with Yuta as a trainer for the Akiba Raid Party, and later becomes a member of the Shibuya Raid Team along with three other members of his guild.
Kozaru dons a typical ninja outfit, with a large green scarf, a purple sash around his waist, and arm bracers. He also has some sort of headgear on his face.
Along with playing similar roles in battle, Kozaru also shares his ninja role-playing theme with Akatsuki. (In fact, his ninja role-playing earned him the nickname "Gozaru" (ゴザル) in D.D.D.) He seems to have formed a sense of camaraderie with her after being her sparring partner in preparation for Operation Capture.
Fallen Guardian arc
Yuta and Kozaru accompany Rieze to the Akiba Raid Party's training grounds to help Akatsuki learn an Overskill. After they knock her down during training, she pushes herself up and tells them to have another go, causing Yuta to look at Kozaru questioningly, who just sighs.
Homesteading the Noosphere arc
With Rieze, Richou, and Yuzuko, Kozaru is one of the four representatives from D.D.D participating in the Shibuya raid. When Shiroe begins panicking, causing the raid party to be in disarray, Kozaru comments on the sloppiness of the whole thing.
Kozaru is one of the D.D.D members that originated from D.D.D Diaries, a Japanese Log Horizon fan fic (and the source for most of D.D.D's members).