Rei Nasu/Plot

Background
Nasu joined Border approximately two years before Yūma Kuga and Chika Amatori as part of a research on whether sickly individuals will produce healthy trion bodies, becoming a proficient Shooter within the following six months. At some point she asked her cousin Narasaka to mentor Hiura.[BBF Q127] On one occasion she had a special feature dedicated to her on TV.[BBF Q266]

B-Rank Rank Battles Arc
On the eve of Nasu Unit's match against Tamakoma Second, Nasu lies in bed while discussing the footage of Yūma's matches against Kō Murakami with Kumagai and Shiki. Together they decide to base the team's strategy on Nasu's firepower. When Hiura arrives, she announces that she could not change her parents' mind about moving. Nasu resolves to reach the top of B-rank within the end of the rank battles season.

Minutes before the match, after revising the strategy, Nasu asks Kumagai and Hiura to cover her and vows to get a lot of points. She spawns on the eastern side of the map with Chika, Osamu, Kuruma, and Betsuyaku. and immediately moves to secure the riverbank. When Chika tears down the bridge, Nasu proclaims that she will defeat every opponent on her side of the map. As she pursues Osamu, who confronted her, she is sniped at by Betsuyaku, who misses due to the rain. With Shiki's assistence, she retaliates, but the Sniper is saved by Kuruma. Nasu fights all three opponents at the same time, breaking through Kuruma and Betsuyaku's guard with a feint and prompting Osamu to double his efforts to pressure her. She manages to take out Betsuyaku with Tomahawk and, since Chika revealed her position to assist Yūma, she switches to hounding her opponents with her mobility, targeting the injured Kuruma first. Right after she fires the fatal round, she is hit by Kuruma's Hound. Osamu moves in to deal the final blow, but having predicted it, Nasu programmed some of her bullets to double back, defeating him. Next she turns to Yūma, but the critical trion loss forces her to bail out before she can honor her promise of scoring four points. She apologizes to her teammates, but they reassure her she fared splendidly against four opponents. They agree to keep trying to climb up the ranking.

Nasu and Kumagai are stationed inside HQ during the the Galopoula blitz. Nasu's Viper forces Wen Sō to split off and hold the two Border agents back. After getting a sense of the durability of the Dogs' shields, Nasu starts taking them out one by one. Wen Sō puts up a smokescreen and disguises herself as Kumagai, getting close to Nasu and managing to cut off her arm. Nasu withdraws behind Kumagai. When the two realize that their opponent relies on her Dogs for defense, Wen Sō activates Servitora and prevents Kumagai from following Nasu into a hallway. Kumagai however spots her mirrors and Nasu begins to shoot them down. As she does, Kikuchihara uses his Side Effect to determine which is the real Wen Sō, allowing Kumagai to cut her. Nasu exploits the opening to focus her fire on her, defeating her.