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 cold, harsh person — being unabashedly blunt towards competitors and colleagues alike, even his former teammate. For instance, 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. Ninomiya holds one's ability to high regard, believing that praise should be reserved for those superior to the complimenter. He is nonetheless highly perceptive in combat, with a slight competitive streak.  Despite his apparent arrogance, Ninomiya is not above politely asking those he considers inferior for help.
(About Kei Tachikawa) "That idiot Tachikawa... Who'd say he'd make a tin-pot analysis like that..." 
(About Osamu Mikumo) "Four-eyes is just a small-fry who gnaws at tactics. He has no chance of winning against someone who can both strategize and fight." 
(About Mirai Hatohara) "Look closely and try to remember. A boring woman with a fake smile on her face." 
(When determining back-up units) "No, not Tachikawa. My squad has no need of him." 
He likes Yakiniku (grilled meat), ginger ale, and people with talent.
Hating the cosplay look, he set the uniform of his squad to black suits not realising that it resulted in the most cosplay-like look.