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. Due to his popularity in HQ, there were ungrounded rumors in Border Headquarters saying he moved to Tamakoma Branch due to his "faction" threatening the Kido and Shinoda Factions.

Black Trigger Capture Arc
Karasuma is first seen playing along with Yūichi Jin when he lies to Kirie Konami about Chika Amatori, Osamu Mikumo, and Yūma Kuga 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 Reiji 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 Marmod.

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 Lampyris 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, noting 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, but with the arrival of Yōsuke Yoneya, Kōhei Izumi, and Shun Midorikawa, they try to continue fleeing. Karasuma leads 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.

Round 1
Following Tamakoma Second's victory against Yoshizato Squad and Mamiya Squad, Karasuma, Konami, and Yōtarō Rindō celebrate with them. Karasuma and Konami talk to them about the higher level of the mid-tier B-rank Squads. Karasuma starts trying to give advice on how to go against Suwa Squad, but is stopped by Kizaki who tells Tamakoma Second to go find the data by themselves. Karasuma thinks this is harsh, but Kizaki tells him that letting them gather information on their own is also a part of their training.

Round 3
Karasuma, along with Konami, Yōtarō, Takumi Rindō, and Hyuse, watches the match from Tamakoma.

Round 4
Before the match, Osamu asks Karasuma to teach him how to fight as a shooter. Karasuma turns him down, telling him that he has yet to reach that level. However, Osamu conveys his determination to learn and he gets in touch with Izumi and Arashiyama Squad, giving Osamu opportunities to work with them. After Osamu works with Izumi and Takeru Yuiga, he asks Karasuma about the latter. Karasuma reveals that Yuiga is the son of Border's top sponsor, and was put into Tachikawa Squad after his insistence on being in an A-Rank squad.

Galopoula's Invasion
Karasuma, alongside Kitora and Jun Arashiyama, back up the snipers from the ground. They pull back briefly when Koskero sends the Dogs up to the roof to distract the snipers, but once Midorikawa and Shinnosuke Tsuji make their way up to the roof, they presumably rejoin the snipers in taking the Idras out. Under Masataka Ninomiya's command, he and the mid-close range forces move to regroup with Shūji Miwa and Yoneya. As they move out, the ground forces encounter the Idras that Yomi is controlling. Karasuma defends against one before Futaba Kuroe and Kitora go after it. After getting the signal from Kōtarō Suwa, Karasuma activates Geist Gunner Shift, and was given 216 seconds. Thanks to this defeats multiple Idras with ease. Afterwards, due to how Geist can't be deactivated, he was forced to bail out after the time limit. as such, he was not present at the end of the battle.

Round 8
Osamu and the rest of Tamakoma Second trains with Karasuma as he takes on the role of Ninomiya. He has Konami take over cooking duty that night, and transforms into "Ninomaru" to help Hyuse and Yūma come up with counter-strategies to Ninomiya's attacks by mimicking him. Later, the two ask him to imitate other agents to which Karasuma does not do. They eat dinner as a branch that night. The day of the final rank war, Karasuma can be seen sitting in the audience, two seats away from Kitora. While Konami is dramatically trying to keep Hyuse's secret weapon under wraps, Karasuma casually tells Kitora that he thinks it is Viper (this turns out to be correct).

When Izumi asks Karasuma about Hyuse's loss, Karasuma says that they came up with a number of plans. However, he seems worried for Chika. By the time Ninomiya and Yuba's duel rolls around, Karasuma has shifted over to be sitting directly next to Kitora to allow Izumi to sit on his other side. They discuss the plans that Tamakoma Second had to counter Ninomiya Squad. Izumi voices his concern, noting once again that Hyuse is no longer in the stage, but Karasuma tells him that Osamu will have to take Hyuse's place. Despite Kitora's doubts, Karasuma believes that Osamu will be able to pull off their plan. When Kurauchi goes off on a trigger tangent, Izumi comes to the conclusion that Chika is unable to shoot at people. Karasuma plays dumb, but is called out on it immediately. He states that while he was worried for Chika's state of mind after her first miss, it seems that it is not affecting her as severely as it would have and he is relieved.

When Osamu sends his Hound (assumed Asteroid) at Ninomiya at the very end of the match, he and Karasuma declare their victory as the audience winces in pity.

A-rank Squad Evaluation
Karasuma along with the majority of the A-rank agents participates in the A-rank Squad Evaluation. He shows concern for Hyuse's situation with his horns, to which Reiji replies that he will check on it with leadership. He also shows confusion when everyone describes Aki Hiyami as level-headed.

Phase 1
At the start of Day 1, Karasuma and Reiji comment on each of the temporary squads' first actions, those actions being: investigating the accommodations, looking at the data, or splitting up. Later on when the temporary squads were discussing their answers for the first special assignment, Karasuma gave points to Ōji and Hana for their speculations.

On Day 2, Karasuma was once again a judge during the special assignment, during which there was an argument between the A-rank agents due to how Murakami, and Kitazoe were handling the situation of needing to pick someone to do the assignment. In the case of Karasuma, he thought there was nothing wrong with Murakami's approach. Later at the end of Day 2, Ōji's Squad 2 has a discussion about the test scores. near the end of said discussion Ōji mentions that the squad probably doesn't need excellent scores as Tachikawa often goes on Away Missions, yet Ōji thinks Tachikawa wouldn't be able to ace the Away Mission Test. Karasuma, as well as Miwa, and Konami all agree with Ōji and each of them gives him a point.

At the start of Day 3, Katagiri, and Yukimaru reviews the Day 2 logs for Mizukami's Squad 9 after coming back from a Defense Mission. afterwards they check out how the other A-rank agents evaluated Mizukami during his and Teruya's argument, where it is revealed that Karasuma was against Mizukami's strategy. Later at the end of Day 3, Katori gets into a bad mood, and barely talks to the rest of her squad. After Osamu talks to the rest of Katori Squad The squad learns they need something to distract Katori, so Oki brings up Karasuma (due to her being a fangirl/has a crush on him). so Suwa brings up the idea of having Osamu ask if Karasuma would be willing to go on a date with Katori, which Karasuma is fine with.