Osamu Mikumo/Plot

Background
He lived in Hasunobe during the time of the First Large-Scale Invasion. Rinji Amatori was investigating Neighbors during the days before he went missing, resulting in Osamu owing him a favor of looking after Chika for him if he wasn't able to anymore. Due to this, Osamu attempted to join Border, but he was not accepted. He tried to sneak in, but a Trion Warrior attacked him. Jin saved Osamu from it.

Introduction Arc
Osamu is first introduced as a normal 9th grade high schooler. He has been shown to have high morals and is quite willing to sacrifice harm to himself in order to save others from harm despite being quite weak. When Yūma arrives as a transfer student, Mizunuma assigns Osamu to be his companion. When the 3 Idiots try to hurt Yūma, he tries to stop them, but they easily overpower him. Yūma manages to fend them off, earning their hatred. After school, Osamu tells Yūma not to fight back, saying that that is the Japanese way, when the bullies return in a larger group, forcing Osamu and Yūma into the Danger Zone. Osamu once again tries to stand up for Yūma, but they thrash him easily, and Yūma defends himself. Just then, a Bamster appears, and tries to eat one of the Idiots. Despite what they did to him, Osamu reveals that he is a Border member and activates his Trigger to try and fight it. However, he is hopelessly outclassed by the beast. Yūma uses his Trigger to assist Osamu and save the bully. Osamu asks Yūma about his identity, and Yūma reveals that the Trigger was his father's, and his father had an acquaintance in Border. Osamu asks how Yūma got his hands on a Trigger, since only a Border agent could have Triggers. Yūma reveals he is a Neighbor. Surprised by the revelation, Osamu leaves the area with Yūma to avoid both of them attracting attention. He walks with Yūma through the city, where Yūma gets into several conflicts as a result of his social naivety.

The next day, the 3 Idiots begin harassing people again, having had their memories wiped by Border. Yūma stomps his foot to scare them off without actually hurting them, earning Osamu's praise. Several students inquire about Yūma's background, where he reveals that he comes from warring zones. Osamu realizes that this shaped Yūma's personality. He stops Yūma from revealing that he is part of Border. When they are alone, he threatens Yūma to not hurt others unjustly, else he would report him to Border. At that moment, Mole Mods attack the school. Osamu uses tries to fend them off, but it easily defeats him, and nearly manages to kill him. Luckily, Yūma saves him. He uses Osamu's Trigger to avoid leaving any evidence of a foreign Trigger being used. Osamu and Yūma's actions prevented any casualties from occurring in the school. Yūma gives the credit to Osamu for defeating the Trion Warriors, thinking that it would be in his favour. However, Arashiyama Unit is forced to send Osamu to HQ for detainment since using training Triggers outside of Border is disallowed. Kitora personally accompanies Osamu to HQ. On the way, an Ilgar attacks the city, which Kitora is forced to fight. Osamu, supported by Replica, helps save civilians. When he is taken by Border, they decide to not take punitive measures since Osamu's actions did help people, and Border's reputation was enhanced by his heroic actions, as well as being defended by Jin himself. After the meeting, Miwa expresses suspicion of Osamu since Osamu's defeat of the Trigger seemed to good, as well as traces of a foreign Trigger being found. Jin states he defended Osamu because his Side Effect told him that he would be useful. Replica explains the mechanics of a Side Effect. Replica acts as a medium for communication between Yūma and Osamu, where Yūma reveals he is investigating the school at night. The following day, Yūma tells Osamu the cause of the Irregular Gates: Rads, capable of opening gates themselves. Jin sees this, very curious about Yūma, but not willing to sell him out to Border.

Osamu has Chika meet with Yūma to identify why she seems to attack Trion Warriors. At the meeting point, she is attacked by a Trion Warrior, but Yūma and Osamu save her. Osamu has been promoted to B-Class, and has his own Trigger. Yūma has Replica take a reading of Chika's Trion, and it shows that Chika has an unduly large amount of Trion, which attracts Trion Warriors. Miwa Unit, having seen this and suspecting Chika of being a Neighbor, order them to halt. Yūma reveals that he is the Neighbor, and engages in a fight with the unit. Yūma tells Osamu to not interfere. Osamu calls Jin and informs him of the situation, but Jin assures him everything would be alright. It turns out as Jin said: Yūma defeats Miwa Unit. Following the battle, he and Jin have to report to Border for hiding the existence of a Neighbor. Here, Kido orders Jin to capture the Black Trigger.

Black Trigger Capture Arc
Osamu is shocked that Jin is forced to arrest Yūma, but pleased when Rindō allows Jin to do as he pleases. Later, Karasawa asks him to tell them the name of his Neighbor acquaintance or his Border-affiliated father. He tells them that it's Yūma Kuga, much to their shock. This makes Osamu wonder who Yūma's father is. Shinoda explains that Yūma's father, Yūgo was a member of Old Border, a senior to Rindō and himself, and a colleague to Kido. When asked about Yūgo's whereabouts, Osamu informs them that Yūgo passed away. Shinoda orders Osamu to stay connected with Yūma. Outside of the conference room, Osamu asks Jin if the fact that Yūma's father was an acquaintance of the higher-ups ensures his safety. Jin responds that he's not sure, and explains the three factions of Border: Kido Faction, Shinoda Faction, and Tamakoma Branch. The three have respective doctrines of never forgiving Neighbors, town's peace takes priority, and Neighbors can be good so let's be friends. Yūma's joining of Tamakoma Branch would tip the balance of power. When Osamu reunites with Yūma, he warns him about his possible pursuers from Border. Jin offers Yūma a simple solution by having him join Border itself, much to Osamu's surprise. When taken to Tamakoma, Osamu notices that it is built on a river, to which Jin responds by reciting the history of the building. When Chika asks Usami about the selection process for the Expedition Force, Osamu is surprised at her interest. After Yūma refuses to join Border, Replica elucidates Yūma's reason for coming to Earth, as well as his background. After this, Replica asks Osamu to give Yūma a goal, as now he has no motivation for living. While Osamu is wondering what the goal could be, he stumbles upon Usami and Chika, the latter of which tells Osamu that she wants to join Border as she wants to take action towards finding her missing associates. Osamu tries to talk her out of it, reasoning that it would be too difficult to get to A-Rank. After hearing from Usami that it is indeed possible to reach A-Rank with the help of Tamakoma First, Osamu consults Chika. Following that, he meets Yūma, and they talk about the fact that he has no reason to stay on Earth. Osamu then asks Yūma to lend his power to their future team, so that they could get to A-Rank and join the Expedition Force. Yūma accepts, but demands that Osamu be the leader. With that, the three are enlisted into Tamakoma Branch.

Border Enlistment Arc

 * Osamu Mikumo vs. Shun Midorikawa: Loss (10-0)

B-Rank Rank Battles Arc

 * Mikumo Unit vs. Suwa Unit vs. Arafune Unit: Win
 * Mikumo Unit vs. Nasu Unit vs. Kuruma Unit: Win
 * Mikumo Unit vs. Ninomiya Unit vs. Azuma Unit vs. Kageura Unit: Loss (Ninomiya Unit's victory)
 * Mikumo Unit vs. Katori Unit vs. Kakizaki Unit: Ongoing