Osamu Mikumo/Plot

Background
Due to his house location, he went to a elementary school in Hasunobe during the time of the First Large-Scale Invasion but has to go to one of Mikado's middle schools. 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 to personally speak to the recruiters, but a Trion Soldier attacked him. Jin saved Osamu from it.

Introduction Arc
Osamu is first introduced as a normal 9th-grade student. 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, Marmods 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 Soldiers, thinking that it would be in his favor. However, Arashiyama Squad 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 too 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 Soldiers. At the meeting point, she is attacked by a Trion Soldier, but Yūma and Osamu save her.

Osamu has been promoted to B-rank 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 Soldiers. Miwa Squad, 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 Squad. 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 Squad. 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 Away Team, 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.

Large-Scale Invasion Arc
Osamu is at his school when the gates open during the invasion. He then instructs the students and teachers to evacuate and asks Natsume and Chika to join them. Later joins Yūma in eliminating the influx of Trion Soldiers during the invasion. When the Rabbits join the skirmish, Osamu is easily overwhelmed by the new model, which forces Yūma to use his Black Trigger for which he is forced to stay inside the Forbidden zone and help in eliminating the other Rabbits as he may be mistaken for being a Neighbour by other agents. Osamu then asks Shinoda for permission to assist C-rank trainees and is accompanied by Kitora.

After reaching the trainees, the group is attacked by a Marmod and a Rabbit. Osamu battles the Mole mod, whilst Kitora battles the Rabbit. Both of them defeat their targets, but from the Rabbit's body, three Rads come out and summons three new Rabbits. Kitora figures out the motive for the invasion and instructs Osamu and the trainees to run away from the scene before she is cubified by one of the Rabbit. After which Tamakoma First arrives at the scene to help the group in evacuating the area. The newly arrived allies easily clear out the Rabbits, but are confronted by two humanoid Neighbours, Hyuse and Viza sent to kidnap Chika for her enormous trion level.

Kizaki instructs Karasuma and Osamu to take Chika and the other trainees to the headquarter. After arriving at a remote entrance to the headquarters the group finds out that all the entrances are in lockdown due to Enedora's invasion of the headquarters, which further forces Karasuma and Osamu to accompany the group straight to the main entrance of the HQ. After which they are confronted by Hyuse and Viza again but Jin and Yūma arrive to defend them so they can evacuate the area.

Osamu, Karasuma, and the trainees move ahead to the HQ's doors, but they are attacked by several Rabbits again. Although initially outnumbered, Izumi, Midorikawa, and Yōsuke arrive to help them evacuate the area. When a Rabbit uses magnetic shards similar to Lampyris to disable Osamu, Chika asks him to use her trion in order to defeat the Rabbit. When the Rabbits are easily overwhelmed by the reinforcement, Hyrein decides to join the battle himself.

Midorikawa is forced to bail out after his trigger is disabled by Hyrein's Black Trigger and he is pinned down by a Rabbit. Many of the trainees are also cubified by Alektor bullets in the confrontation. Izumi then orders Osamu and Chika to flee the scene, while he distracts Hyrein. But Osamu instead decides to attack Hyrein by using Chika's trion again, but fails to do any damage because of his lack of skills, and Hyrein cubifies Chika with Alektor. Osamu takes the cubified Chika and then runs for the HQ, but is chased by two Rabbits and pinned by one of them. Replica arrives and uses his Shield to protect Osamu and then tells him that Yūma asked him to accompany Osamu to the HQ. Replica then creates a Rabbit of his own to defend them against the Aftokrator's Rabbits.

Osamu and Replica then try to move further to the HQ but then are confronted by Hyrein who had arrived at the scene after defeating Izumi and Karasuma. Replica then creates an extension of himself and sends it ahead to the HQ entrance to force it open. Replica and Osamu then try to use the buildings to shield themselves from Hyrein's bullets. Replica's extension is then destroyed by Mira and she uses the marker on Osamu's hands to track them down and opens a gate near them. Osamu is then hit on one of his legs by Alektor's bullets sent through the gate and is partially handicapped. Just then Miwa arrives and Osamu asks him to take Chika's cube to the HQ, but Miwa kicks him instead and orders him to do it himself. He also further tells Osamu that he only came to defeat Hyrein because of his hatred for Neighbours and not to save Osamu.

While Miwa fights Hyrein, Osamu tries to attack Hyrein but discovers that his Asteroid is dysfunctional because of his channeling of Chika's trion. Osamu then runs to the entrance where he is confronted by Mira. To defend them Replica summons his Rabbit again, but then is cut in half by Mira in the process. Replica then destroys the marker on Osamu and tells Osamu that even though he is bisected and lost most of his functions, he will still be able to force the HQ doors open but the analysis of the door is not complete so they'll have to use a different plan.

Osamu then further runs up to the HQ but is impaled by Mira's spikes. Hyrein then sends Alektor bullets to cubify Osamu, but Osamu then deactivates his trigger and assumes his normal body, which negates the Alektor's ability. Osamu continues towards the base and is impaled by Mira again, but this time on his real body and is hence gravely injured. But Osamu then uses Replica's plan to try to force the enemies to retreat by throwing Replica inside the enemy ship. Replica then overrides the system and sets it to auto-retreat in 60 seconds. Hyrein desperate to capture Chika's cube then uses Mira's portal to teleport Miwa out of range and rushes at Osamu. Yūma then arrives to help Osamu, and both him and Miwa with Fūjin equipped uses Replica's extension to locate Hyrein and attacks them from afar. Osamu then falls down on the ground. Hyrein then takes Chika's cube from him but discovers that it's just a regular trion cube and not Chika. With no time to look out for Chika again, he orders Mira to retrieve Viza and prepare for retreat signifying the end of the invasion.

Osamu is later found by Yoneya and Natsume, lying unconscious on the ground in a pool of blood, and is then taken to a Hospital.

Round 4
Prior to the rank battle, Osamu visits both Arashiyama Squad and Izumi to aid in his training. After being manhandled by Yuiga, Osamu asks Izumi to teach him how to make composite bullets, but Izumi does not believe he is ready. The two strike up a deal: if Osamu can defeat Yuiga 100 times, Izumi will teach him how to use composite bullets. Later that evening, Osamu asks Karasuma as to why Yuiga is an A-Rank, to which he is told that Yuiga got nepotism privileges. Chika tells Osamu and Yūma that Ema knows her weakness.

Osamu is teleported into the map near Okudera. He escapes Kitazoe's bombardment before coming face to face with Inukai. Osamu lures in him into a building, and is able to defend against the gunner's attacks until their confrontation is interrupted by the arrival of Koarai, Okudera, and then later Tsuji. As he is about to attack, Azuma shoots him through the walls of the building making him the first casualty of the match. Osamu is seen feeling ashamed for not being able to put any of his new skills to use.

As per rank war rules, Osamu is able to communicate with Chika and Yūma through Usami's comms. When Chika shoots, he is shocked by her action. Unfortunately, this draws attention to her and Osamu commands her to bail out. Although she had trouble at first, Chika is safely able to bail out. Osamu, Chika, and Usami observe the match until Yūma's subsequent bail out.

Following the match, Kazama notes that Osamu needs to find a better solution than self-improvement, and should act more like a captain. That evening, he asks Jin to join Tamakoma Second.

Border Enlistment Arc

 * Osamu Mikumo vs. Shun Midorikawa: Loss (0-10)
 * Osamu Mikumo vs. Sōya Kazama: Loss (0-24-1)

B-Rank Wars Arc

 * Tamakoma Second vs. Suwa Squad vs. Arafune Squad: Win
 * Tamakoma Second vs. Suzunari First vs. Nasu Squad: Win
 * Tamakoma Second vs. Ninomiya Squad vs. Kageura Squad vs. Azuma Squad: Loss (Ninomiya Squad's victory)
 * Tamakoma Second vs. Katori Squad vs. Kakizaki Squad: Win
 * Tamakoma Second vs. Ōji Squad vs. Ikoma Squad: Win
 * Tamakoma Second vs. Kageura Squad vs. Azuma Squad vs. Suzunari First: Win
 * Tamakoma Second vs. Ninomiya Squad vs. Ikoma Squad vs. Yuba Squad: Win