Koinuma is a light-skinned young man of average height and build. He has middle-part, straight sandy hair that covers his ears and reaches to his chin. Like the other members of his unit, in his trion body he wears dark pants, a lighter jacket with a high collar, and boots of the same color, as well as dark-rimmed goggles with square, protruding lenses.
As a B-rank agent, Koinuma should be more skilled than a C-rank trainee. His unit's foremost tactic is a combined attack which sees the three Shooters focusing their Full AttackHound on a single target. This technique, called Hound Storm, was extolled by Ken Satori as a very powerful attack. However, the whole unit was easily defeated by the combined might of Chika and Yūma without managing to put up any form of resistance.