At some point before the start of the series, his Sniper, Mirai Hatohara, had illegally supplied Triggers to civilians. She had gone missing on the same day as Rinji Amatori, leading him to believe that she may have been instigated by him. Due to the breaking of Border's rules, his team was demoted to B-Class.
Ninomiya appears after Tamakoma Second's battle with Suzunari First and Nasu Unit discussing the battle with Izumi. He is harsh of Tamakoma Second, believing that Osamu and Chika are weaklings who only have strategy and luck. He later visits them, asking them for information about Rinji Amatori, as well as telling them the story of Hatohara.
Later, his team is up against Osamu's in a Rank Battle. He attacks Kitazoe with a pyramid-shaped Asteroid. Ema attempts to snipe Ninomiya, but the latter easily blocks it. Ninomiya orders Inukai and Tsuji to go hunt down Tamakoma Second while he hunted down Kitazoe and Ema. He did so with relative ease, and then arrived at Yūma and Kageura's battle where he defeats Yūma and then hides from Kageura and Azuma. The three remain in a deadlock with nothing happening until a time out is called, with Ninomiya Unit having the victory at 3 points in total.
During the Galopoula Invasion, he assumes a leader role and directs the defence of HQ.