Ninomiya is a tall man with brown eyes and brown, neck-length hair with a middle-parted fringe. His Trion body uniform is a black three-piece suit, of which he leaves the jacket unbuttoned.
Ninomiya comes across as a harsh person, being extremely blunt towards everyone even towards his former teammate. He is highly perceptive in combat and also has a bit of a competitive streak.  Ninomiya has a very strong view on compliments, believing that one should only compliment those who they believe to be superior to themselves. He openly told Tamakoma Second that their only chance of personally recovering Rinji was to get Yūma into A-rank and send him alone to the Neighborhood. Despite his apparent arrogance, though, Ninomiya is not above politely asking those he considers inferior for help. 
Mirai Hatohara: Ninomiya has very little respect for his former teammate, possibly due to a mixture of her inability to shoot opponents directly, and her breaking Border's rules by giving triggers to civilians, which demoted them to B-rank. It seems he still cares about her, however, as when there is a lull during the round 4 battle, there are 5 snowmen seen around him, totaling the number of members in his unit including her.
Kōhei Izumi: As top-tier Shooters, both of them have respect for each other and their opinions. Despite being the No.1 shooter at the time, Ninomiya asked Izumi to teach him techniques he lacked.