Kumagai joined Border roughly two years before Yūma Kuga and Chika Amatori. She was scouted by Yūko Kumagai and, at Rei Nasu's request, Tōru Narasaka accepted her as one of his disciples. Before Chika and Izuho enlisted, she was the youngest female Sniper, trainees included.
Four days later, they fight a rank battle against Suzunari First, Urushima Unit and Kakizaki Unit. They force their opponents into a melee and manage to score two points, but are unable to defeat Kō Murakami, marking their sixth consecutive loss against Suzunari First.
Following Aftokrator's invasion, Hiura's parents decide to move out of Mikado City as soon as she graduates from middle school. Hiura's attempts to dissuade them cause her to miss the strategy meeting two days before her unit's match against Tamakoma Second and Suzunari First. However, she is unable to change her parents' mind, as she tearfully informs her teammates the next day. They resolve to climb up in the ranks before Hiura has to leave the unit.
She is in a better state of mind the following day as the unit goes over their strategy a few minutes before the start of the match. She spawns on the western side of the map with Kumagai, Yūma and Kō Murakami. Hiura does not activate Bagworm on account of the low visibility and strong wind and rushes toward the bridge. When she has almost reached it, however, Chika demolishes it with her Ibis. Kumagai thus tells Hiura to go into hiding as she faces off against Yūma and Murakami. After reaching a sniping position she asks Kumagai which of her two opponents she should shoot first, her teammate telling her to take the easiest shot and to wait for a moment when they cannot afford to look out for her. At Kumagai's signal, Hiura opens fire against Yūma, who however manages to dodge with Grasshopper and dashes towards her. Against Kumagai's judgment, she refuses to bail out and instead shoots at the immobile Meteor cubes she had placed on the street beforehand, causing two buildings to collapse on Yūma. She switches to Ibis to take him out as soon as he steps out of the dust cloud, but he distracts her by using his own arm as bait before closing in and destroying her trion supply system.
After the end of the match, she congratulates her captain on her performance while feeling down about her own. She stops crying when Yūichi Jin praises her and Kumagai's decision not to bail out. The members of Nasu Unit resolve to keep trying to move to the top tier of B-rank.
A few days later Hiura befriends Chika and Izuho Natsume when they show up for joint Sniper practice with a cat. While she agonizes over whether she wants to be petted, the cat climbs on top of Isami Tōma's head, making Hiura envious. After the end of the drill Hiura proudly tells Chika and Natsume that Narasaka is her teacher. She then congratulates him on ranking first again, but he points out that Tōma and Yuzuru Ema were not really trying, showing her, Chika and Natsume the figures they drew on their targets. Hiura praises Ema for his accuracy and introduces him to Chika before leaving for a squad meeting.
Some time later Hiura participates in stealth and acquisition joint Sniper training, sighing to her mentor when she ranks 11th. On February 19, she is one of the Snipers stationed atop Border's base in preparation of Galopoula's attack. She is surprised to see humanoid Trion Warriors. At Reiji Kizaki's signal she starts shooting, but she has to stop when Koskero starts summoning Dogs on the roof. Narasaka protects her against their bullets. With Kizaki, Tetsuji Arafune, Shun Midorikawa and Shinnosuke Tsuji fighting back the Dogs, the Snipers resume firing at the Idras. Hiura remains on the roof while the Snipers led by Kizaki provide assistance to the agents on the ground. After the Galopoula forces retreat, Hiura ecstatically shares with Taichi Betsuyaku that Nasu and Kumagai beat an enemy agent.
During the Radar search drill Hiura practices with Natsume's cat on her head. Later, when she overhears Kōji Oki say that he has trouble sniping pretty girls, she asks him about herself, and rejoices when he claims that it would be difficult for him to shoot her. She, Chika and Natsume keep practicing together until 6 p.m..