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.

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 Osamu, Chika, Kuruma, and Betsuyaku. She pursues Osamu while blocking attacks from Betsuyaku and Kuruma and attacking them. After Chika destroys the bridge, she changes her plan such that she would eliminate all her opponents on her side. She manages to defeat all 3 of her opponents, but runs out of trion before she can fight Yūma. She regrets not being able to win for her team, but her team assures her that they did well.

During the Galopoula Invasion, she and Kumagai confront the three intruders, and Wen Sō stays behind to face them. Nasu and Kumagai overwhelm her with their respective firepower and swordsmanship, and Wen Sō uses a smokescreen to counter them. She then disguises as Kumagai to deceive Nasu, and manages to cut off her arm. With the help of Kikuchihara's Side Effect, they manage to defeat her.