Yūko Kumagai/Plot

Background
Kumagai joined Border roughly two years before Yūma Kuga and Chika Amatori. She is the driving force behind the creation of Nasu Unit, having scouted Sayoko Shiki and Akane Hiura.[BBF Q127]

Introduction Arc
Nasu Unit was presumably involved in the extermination of all Rads in Mikado City.

B-Rank Rank Battles Arc
On February 1 Nasu Unit is assigned to defense duty. 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. While Kumagai is taking a drink from a vending machine, Yūichi Jin gropes her, causing her to punch him. By way of apology he tells her that two of her upcoming opponents, Yūma and Murakami, are having a solo match. She manages to secure the footage and the next day she shows it to Rei Nasu and Shiki. The three discuss Yūma's abilities with respect to his teammates, recognizing him as the ace of Tamakoma Second. Kumagai is more concerned about Suzunari First, against which they have lost six times in a row. Shiki remembers that Suwa Unit defeated the team by remaining in Gunner range, so they decide to base their strategy on Nsau and pick a stage that facilitates it. The following day, Hiura tells them that she was unable to dissuade her parents from moving after she graduates middle school.

One day later, on Saturday February 8, Kumagai recaps the unit's strategy minutes away from the start of the match. She spawns on the western side of the map with Hiura, Yūma and Murakami. She is the first to reach the bridge, which she plans to secure until Hiura passes, then follow her and blow it up. She prepares to clash for Murakami, but just as he arrives, Chika demolishes the bridge with Ibis. Kumagai realizes that it is because he had foreseen that scenario that Jin told her about Yūma and Murakami's match. She and Murakami clash, with the no. 4 Attacker quickly gaining the upper hand. Yūma attempts to throw him into the river, but he catches himself with Thruster. Kumagai instructs Hiura to take out whichever of the two is easiest to hit.

The three-way fight moves to the bank, with Kumagai advising Hiura to shoot when either Murakami or Yūma make a big move. When Murakami does so, Yūma lures Hiura's shot and rushes to take her out, leaving Kumagai by herself against Murakami. Fearing that Yūma will cross the bridge, she and Murakami move towards it. She decides to fight the latter before Yūma can target her too, but she has her left arm quickly cut off. To make up for it, she starts bombarding Murakami with Meteor, intending to keep out of his range until his Raygust breaks. However, he propels it at her just as she fires, causing her to self-destruct, as well as avoiding the trap she left behind. A defeated Kumagai cries in her unit's operation room.

She has recovered by the time Nasu bails out, asking her how she feels and congratulating her on how she fought. In his analysis Jin praises Kumagai and Hiura's decision not to bail out against superior opponents, which allowed Nasu to defeat the enemies on her side. The members of Nasu Unit resolve to keep trying to move to the top tier of B-rank.

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. Kumagai focuses on keeping the Dogs and Wen Sō herself away from Rei. When the Neighbor puts up a smokescreen, Kumagai is pinned in place by the Dogs, prompting Nasu to come to her aid; however, she positions herself behind a disguised Wen Sō, and only thanks to a confused Kumagai's call does she manage to avoid lethal damage. 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 struggles to deflect her attacks and takes considerable damage, but she manages to spot the mirrors that Wen Sō is using to create her holograms. While Nasu begins to shoot them down, Kumagai assumes that the mirrors flip Wen Sō's image around its vertical axis and prepares to attack a duplicate, but Shirō Kikuchihara tells her which is the real Wen Sō, who had used her transformation Trigger to flip her own image around beforehand andalso copied some of her copies instead of herself directly. Kumagai manages to cut her, giving Nasu the time to finish her off.