Kyōsuke Karasuma/Plot

Background
Karasuma joined Border about 3 years ago, and was originally a member of Tachikawa Squad. About 1 year prior to the story, Karasuma and Shiori Usami transferred to Tamakoma Branch for unknown reasons.

Black Trigger Capture Arc
Karasuma is first seen playing along with Yūichi Jin when he lies to Kirie Konami about Chika, Osamu, and Yūma being his siblings. Jin tells the members of Tamakoma First that they will be training the three newcomers, and Karasuma is left with Osamu after Yūma is claimed by Konami and Chika is assigned to Kizaki. He and Osamu go to Tamakoma's Training Room 001, where he tells the younger to come at him at full strength.

After Osamu is thoroughly worn out Karasuma questions his ranking, saying that he is weak for a B-rank. Once Yūma and Konami go back into their training room for another round, Karasuma asks Osamu if he's up for one more round. Later, when Konami asks him about Osamu, Karasuma admits that he doesn't really have high expectations for him. When Yūma asks him for a battle, Karasuma tells him that he's busy trying to figure out how Osamu can become stronger. The arc closes with Karasuma changing Osamu's training regimen at his request of becoming a gunner instead of an attacker.

Border Enlistment Arc
Karasuma arrives at the training room after Yūma defeats the simulation Neighbor, greeting Ai Kitora before telling her to help Osamu. Alongside Yūma, the members of Kazama Squad, and other agents, Karasuma watches Osamu and Sōya Kazama have a mock battle.

It is also revealed that Karasuma was the one who talked Osamu into being a Shooter, rather than a Gunner.

Large-Scale Invasion Arc
In a flashback, Karasuma tells Osamu to counterattack instead of only defending. He explains the process of counterattacking for Osamu. Back in the present, Osamu remembers Karasuma's advice and uses it to take down a trion soldier.

Soon after Kitora is turned into a cube, Karasuma arrives at the scene with the rest of Tamakoma First. He enters the battle by using Escudo to protect Osamu from being hit by a trion soldier. Osamu warns Karasuma of the Rabbit's ability to turn agents into cubes, but Karasuma informs him that he already knows, and that engineers at Headquarters are trying to figure out a way to undo the transformation. Hyuse and Viza appear in front of them, and Karasuma and Kizaki move in to take the two Neighbors on. Kizaki orders that their squad prioritizes the safety of the C-Rank agents before they continue battling Hyuse. When Hyuse hits Chika with Lambris and starts bringing her towards him, Karasuma quickly shoots at him to cause Hyuse to encase himself with his trigger. This allows Kizaki to punch Hyuse, making him drop Chika. When Kizaki tells Karasuma and Osamu to return to the base with the C-Ranks, Karasuma questions his orders but Kizaki tells him that he and Konami can take the two Neighbors on.

Karasuma and the others reach an entrance to the base following Enedora's invasion and are unable to get inside. When Viza and Hyuse arrive to their location, Karasuma looks in shock, saying that it hadn't been very long since Kizaki's bail out. He tells Osamu to regroup with Jin and moves in to take on the two Neighbors, but is interrupted by Jin and Yūma's abrupt entrance. Karasuma and the others continue directly towards Headquarters, and Usami informs them of Enedora's infiltration. Karasuma says that they will continue towards Headquarters, because Jin told him to do so and that must mean that there is a reason for that action. The group is confronted by more Rabbits, and they move to escape. With the arrival of Yōsuke Yoneya, Kōhei Izumi, and Shun Midorikawa, they try to continue fleeing. However, the Rabbits overpower the agents and they are forced to take a stand against them. Karasuma continues leading the C-Rank agents away, but they are pursued and attacked by Hyrein. Once Chika is turned into a cube, Karasuma and Izumi tell Osamu to keep running towards the base.

Izumi makes the decision to bail out before he is turned into a cube by Hyrein and his trigger. He apologizes to Karasuma, who thanks him for buying them time. After Usami tells him that he need to buy Osamu three more minutes, Karasuma activates Geist Blade Shift. This gives him 200 seconds until his inevitable bail out.

Flashback to before the invasion, Karasuma confronts Jin and asks him for advice about what to do. Jin tells him that he will be defeated by someone, and then proceeds to ask Karasuma if he feels down and that he shouldn't ask about his future. In the present, Karasuma disagrees with Jin, believing that he was able to avoid participating in big battles until the very moment. Karasuma works to distract Hyrein, and with help from Yoneya he strikes him. However, he finds that he doesn't do any damage and instead breaks his own blade thanks to Hyrein hiding Alektor under his cape. Mira appears when they have less than a full minute left, and the two neighbors reveal that they knew where Osamu was all along. Karasuma attempts to stop them, but is hit by Alektor bullets from behind courtesy of a coordinated attack between the two Neighbors. Karasuma bails out before he turns into a cube, and voices his frustration at not being able to defeat Hyrein.

Karasuma appears in another flashback, when he explains the properties of Lead Bullet to Osamu.

While Osamu is in the hospital recovering from his wounds, Karasuma is seen visiting with the rest of Tamakoma Branch. It is mentioned by Usami that Karasuma, along with Kizaki and others, were awarded for their contribution to Border's victory against the Aftokrator invaders.