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.

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 then. Later joins Yūma in eliminating the influx of Trion Warriors 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 agents, and is accompanied by Kitora.

After reaching the C-rank agents, the group is attacked by a Mole Mod 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 C-rank to run away from the scene before she is cubified by one of the Rabbit. After which Tamakoma First arrives to 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 Neighbouts, Hyuse and Visa sent to kidnap Chika for her enormous trion level.

Kizaki instructs Karasuma and Osamu to take Chika and the other C-rank agents 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 Enedra'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 Visa again but Jin and Yūma arrive to defend them so they can evacuate the area.

Osamu, Karasuma and the C-rank agents move ahead to the HQ's doors, but they are attacked by several Rabbits again. Although initially outnumbered, Izumi, Midorikawa and Yoneya 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, Hairein decides to join the battle himself.

Midorikawa is forced to bail out after his trigger is disabled by Hairein's Black Trigger and he is pinned down by a Rabbit. Many of the C-rank agents are also cubified by Alektor in the confrontation. Izumi then orders Osamu and Chika to flee the scene, while he distracts Hairein. But Osamu instead decides to attack Hairein by using Chika's trion again, but fails to do any damage because of his lack of skills, and Hairein 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 Hairein who had arrived to 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 Hairein'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 Hairein because of his hatred for Neighbours and not to save Osamu.

While Miwa fights Hairein, Osamu tries to attack Hairein but discovers that his Asteroid is disfunctional 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. Hairein then sends Alektor bullets to cubity 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. Hairein 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 Hairein and attacks them from afar. Osamu then falls down on the ground. Hairein 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 Visa 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.

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