Kotarō Tomoe/Plot

Background
Three months after Border moved to its publicly known HQ, Tomoe saw Kuniharu Kakizaki on television and thought he was cool. He enlisted about two years later, which caused him to become the first elementary schooler to join Border. He jumped at the chance when she discovered that Kakizaki was creating his own unit and applied immediately.

Large-Scale Invasion Arc
Along with the rest of his unit, Tomoe joins the fight against Lamvanein. The Neighbor forces them into hiding until Kōhei Izumi, Shun Midorikawa and Yōsuke Yoneya join the fight. At Haruaki Azuma's command, the members of Kakizaki Unit start firing at Lamvanein to pressure him into revealing openings. When the Neigbor targets Tatsuya Kuruma and anticipates Yoneya's ambush, Tomoe, along with Kuruma, Midorikawa and Tsuneyuki Okudera casts his Shields in front of the Attacker, allowing him to close the gap and defeating Lamvanein. Kakizaki Unit is subsequently deployed to defend the southern area of the city. To the best of Tokieda's knowledge, it was the only B-Rank unit to survive Aftokrator's invasion without a single bail out. It was also the recipient of an Excellent Distinguished Service award.

B-Rank Rank Battles Arc
On the day of the match against Tamakoma Second and Katori Unit, Kakizaki has his teammates learn all the building locations of their map of choice and warns them against fighting Yūma or Katori if the three of them are not together. They laugh when he answers Ui's question about his impression of Yūma by saying that he seems a nice guy. Tomoe is amazed by Tamakoma Second's resolve to make the next away expedition despite having only recently joined Border. The three fighters agree to meet up before engaging any opponent when they spawn, and they do so, ignoring the clash between the two enemy aces. When Kakizaki Unit does confront the enemies, Kakizaki and Fumika begin shooting to pressure Katori Unit, while Kotarō flanks them. However, Kakizaki Unit is forced to retreat when they are lured into a position between the opposing teams. Kakizaki Unit and Katori Unit both decide to fight each other and lure Tamakoma Second out of the wire zone, but just then Chika starts destroying buildings with Ibis.

Kakizaki lets Katori Unit swarm the Spider nest first, since that makes it more likely that they will be targeted by Yūma; however, he is the one who comes to intercept them. The three manage to drive Yūma to an open area, but Yūma stays close to Kakizaki, preventing Teruya and Tomoe from shooting at him, until they switch to their Attacker Triggers, at which point he steps away again. Kakizaki and Tomoe begin cutting the wires, but then Tomoe attempts to pin Yūma in place; however, he is propelled into the air, where Chika hits his arm with Lead Bullet. Pinned to the ground, Kotarō becomes the first to bail out, beheaded by Yūma. He apologizes to Ui, but she responds that there was nothing that he could have done.

After the end of the match Kakizaki apologizes to his teammates for being overly cautious and not letting them do as they suggested, but Tomoe objects that if they had charged recklessly, they would have been stopped by the wires and Lead Bullet. Together the members of Kakizaki Unit decide to create two-to-one tactics to complement their captain's signature three-man formation.

During the next enlistment day, Tomoe and Yūta Miura hear about a newcomer who made B-Rank in less than a day. Noboru Koarai is excited at the idea of fighting him, but Tomoe points out that he still has only one Trigger. Tomoe, Miura, Koarai and Hisato Sasamori propose to fight 5 rounds against Hyuse each using only their Kogetsu, and he accepts. Tomoe loses all 5 rounds and is amazed with his skill, and even more so when he bests Shinnosuke Tsuji. When Hyuse duels Tatsuhito Ikoma, Tomoe and Tsuji bet that Hyuse will win, since Ikoma cannot use Senkū, and together with the others watch the match with bathed breath. When it ends, Tomoe joins the throng of Attackers in congratulating Hyuse.