Tamakoma Second

"From now on, you are a team. With this team, you'll be promoted to Class-A, and then, will aim to be recruited to the Expeditionary Force."

- Takumi Rindō

Mikumo Unit, more commonly known as Tamakoma Second, is the B-rank #2 unit in Border. It is led by Osamu Mikumo.

Introduction Arc
Osamu, Yūma and Chika were invited to Tamakoma branch by Yuichi Jin, after asking Yūma to join Border from orders given by Takumi Rindo to retrieve Kuga's Black Trigger. Once at the headquarters, they meet Yotaro Rindo and Shiori Usami who explain Border's regulations, more details about its ranking system as well as more details about Tamakoma. Yūma and Osamu are then called to meet with Rindo. After hearing that Soichi Mogami, the person that Yūma was trying to meet had died, Rindo asks Yūma to join border to which Yūma denies.

Moments later, Replica explains Yūma's reasons to coming to earth: meeting with Mogami Souchi in an attempt to revive his father. He then tells Osamu about Yūma nearly dying and his father sacrificing his life to save him at the last minute. Yūma tells Jin that he plans to head back to his home because living on earth as a neighbor would just give him trouble. Replica then tasks Osamu with giving him a new will to live and to join Border as an agent.

Before fulfilling the task, Osamu is pulled away by Shiori to hear that Chika wanted to join Border in hopes of finding her brother Rinji along with her friend, Aoba. Though indecisive at first, the two accept her decision as Osamu then heads up to Yūma. During the conversation, Osamu asks Yūma to join in hopes to help bring back Chika's comrades then asks for his assistance. The two talk about Osamu's selflessness, then Yūma soon agrees to help Osamu and Chika, under the condition that Osamu would be the captain of their squad. To Osamu's shock and exclaiming how he is far more inferior to Yūma, He simply explains that Osamu being captain is the way that it should be. After hearing the good news, Rindo assigns Osamu, Yūma and Chika to being a squad.

Black Trigger Capture Arc
The next morning, Shiori and Jin give Osamu, Yūma, and Chika the rundown on the Border enlistment and its ranking system. With Yūma eager to get started with battles, Shiori explains that there are specific dates in the year where all new agents are initiated into Border, much to Yūma's disappointment. Jin also explains that Yūma could not use his Black Trigger due to the Triggers being considered S-Rank Triggers, which are prohibited from mock battles. Shiori then asks Chika what position is best for her to which Yūma answers to her being a combatant owing her to her massive trion levels. Shiori begins to explain the team positions based on their combat ranges to determine the best role for Chika as a combatant. Osamu vouches for a doubtful Chika, saying she has a lot of endurance, patience and flexibility. As Shiori starts speaking for her, Jin interrupts her using his side effect, saying that she would be best suited for sniping.

Suddenly, the three remaining A-Rank agents arrive: the gullible and hot headed Kirie Konami, laid back and casual Kyōsuke Karasuma and the calm and stern Reiji Kizaki. Jin explains to the Kizaki Unit that he needed them to be their respective mentors with Konami first choosing Yūma, Shiori suggesting Chika to be taught by Reiji and Karasuma picking Osamu by default.

Later on, Shiori teaches Chika about Triggers and Trion bodies, as well as giving Chika a simulation room for her and Reiji to begin their training. In the next room, Karasuma and Osamu as well as Yūma and Konami use the next rooms to start there respective training. During the training, Konami defeats Yūma multiple times with Yūma scoring the last victory much to everyone's disbelief and Chika shot down a plethora of targets without a single break too Reiji's shock. Three days later, the three continued to train for the enlistment day, Yūma and Chika were greatly praised for their talents and abilities whereas Osamu was ousted for his drastic need to improve despite being a B-Rank. During the battle between Yuichi Jin and the Arashiyama Unit against Tachikawa Unit, Kazama Unit and Miwa Unit, Shiori goes into more depth about combatant types. Finally, after the long bout, Jin giving Fujin to Border and Osamu choosing to become a gunner, January 8th arrived and the trio arrived at Border for official enlistment.

Border Enlistment Arc
As Tamakoma Second, Osamu entered with unease due to their goals: quickly rise to A-Rank, complete the expedition exam, and travel to the neighbor world. In the crowd, they are welcomed by general manager Masafumi Shinoda and Arashiyama Unit, who the latter quickly split into two groups for Attackers and Gunners with Arashiyama and Kitora in one group and Snipers in the other group with Ken Satori. Osamu, Yūma and Chika split into their respective groups Arashiyama explains the ranking system and how promotion works, giving Yūma much more determination than before.

During the simulation training, Yūma sets a record in the Bamster training by finishing the exam in less than 0.6 seconds; when asked to do it again, finishing in 0.4 seconds, astounding everyone. Meanwhile, Chika meets Izuho Natsume and learns more about the sniper triggers. Satori suggests for Chika to demonstrate the power in the Ibis rifle and due to her trion levels accidentally blasts a hole through the Border base. Later on, Sōya Kazama challenges Osamu to a mock battle, to which Osamu accepts without hesitation; only to lose 24 times. Kazama wonders about why Jin sacrificed Fujin for the sake of Osamu's team and to his utter surprise, Mikumo is prompted to try one more mock battle with Kazama. Osamu impressively tied with Kazama in the final encounter. Kazama called Osamu weak but commended him for his creativity, and to Yūma's disappointment to not having a fight with him, Kazama tells him that he is still a C-Rank and will need to rise up for a spar.

The three later reunited after the commotion about Chika's accident and commander Motokichi Kinuta's appearance and sheer praise for Chika's abilities. During Yūma's process of gathering points, Rumors soon speculate about Osamu's battle favoring himself and eventually draws the attention of Shun Midorikawa, who challenges Osamu to a series of battles and easily defeats him. With Yūma angered at Midorikawa for intentionally degrading his captain, Kuga sets a wager for him. Had Midorikawa lost, he would have to properly address Osamu every time they talked to each other. Yūma defeated Midorikawa 8 to 2 and gained the respect of Midorikawa, as well as Osamu finally ridding the rumors of defeating Kazama. In the aftermath, Jin and Yotaro summons Osamu and Yūma to see commander Masamune Kido.

In the meeting, Yūma, Osamu and Jin meet with the Border commanders to discuss the possibility of a neighbor ambush. Replica appears with all known data about the neighborhood but insists the guarantee Yūma's safety so long as he is with Border to which Kido complies. In the mean time, Chika and Izuho continue their training with Izuho envying Chika's progress to which Chika responds with helping Izuho become a better sniper. After the meeting, Shinoda communicates with Yūma and Osamu with much praise with the information given and suggests that Yūma could be immediately promoted to B-Rank. Kuga denies and prefers to rank up normally and track more attention to himself. The next day at school, Border's alarm sounds and Tamakoma Second quickly rushes to the scene.

Large-Scale Invasion Arc
Osamu instructs Chika and Izuho to help with evacuating the civilians as Yūma, Osamu and Replica head off to help fend off the invaders, however, during the battle, the two run into the new type of trion soldier: The Rabbit. Replica immediately informs Osamu that Border is trapped between dealing with the Rabbits stealing Border agents and Marmodsattacking the civilians. Replica suggests to rendezvous with other Border members to attack in designated areas, leaving Osamu worried about Chika and the unguarded civilians. Osamu is immediately assaulted by a Rabbit, which was quickly dispatched of by Yūma, who swiftly switches to his Black Trigger. A nearby Chano Unit sees the transformation attacks Yūma who learns that he's a neighbor but was attacked by the returning Rabbit. It was then destroyed by the arriving Arashiyama Unit. Jun struggles to contact headquarters because of the Ilgar's onslaught of the base that was subdued by Tachikawa. Finally gaining communication, Osamu requests that they meet up with Chika and the civilians to protect them, but Commander Kido interrupts and instructs for Osamu to go by himself after Kuga activated his black trigger, not only due to civilians possibly mistaking him for a threat like Chano Unit, but because Kido needed him to help destroy the Rabbits. Kitora requests to accompany Osamu, which is accepted. As such, Tamakoma Second and Arashiyama Unit split up to their respective duties.

Meanwhile, Chika, Izuho and the other C-Rank agents lead the civilians to safety but were targeted by a rampaging Bamster that was destroyed by Kōda Unit. Little did they know, a black Rabbit who was hiding in the Bamster quickly breaks out and begins to attack by was stopped by the arrival of Kitora and Osamu. A Marmod approaches to attack the C-Ranks. Kitora tells Mikumo to defeat the Marmod much to Osamu's hesitance. After reflecting on his past experiences and training, he destroys the Marmod, then quickly goes to aid Kitora but is blinded by Kitora's speed and offense over the Rabbit. Once the trion soldier is defeated, a Rad escapes from the hull and summons three more Rabbits that quickly defeat a wounded Kitora who reveals that Aftokrator's motives were to retrieve the C-Rank agents. As everyone began to retreat a frightened Chika stayed put in front of the soldier that was shot by Izuho that was captured by one of the Rabbits. Chika immediately reaches for the Ibis rifle and blasts the the soldier, destroying it with help from Mikumo. Osamu quickly informs headquarters of the Neighbors intentions who send in Tamakoma First to aid in helping the C-Ranks. Hairein gathers interest in Chika's trion levels and dispatches Lamvanein, Enedra, Hyuse and Viza to join in battle. Reiji instructs for Osamu to protect Chika as they fend off Hyuse, Viza and the Rabbits.

During the battle with Kazama Unit and Enedra, Yūma and Arashimyama Unit destroys two rabbits and was interrupted by Yuichi Jin who needed Yūma to help prevent the future outcome where Osamu dies. Jin planned for Osamu and Chika to head towards the edge of the forbidden area as Jin and Yūma begin to make their next move. After the battle with Izumi, Midorikawa, Yoneya, and Lamvanein, Jin senses a shift in the future as they advance. Osamu, Chika and Karasuma lead the C-Ranks to the secret entrance only to find that the door is jammed because of Enedra's meddling at HQ. Chika's side effect picked up Viza and Hyuse honing them down. As Karasuma prepared to engage in combat, Jin and Yūma arrive in time to protect them. Osamu was struck by Hyuse's magnets protecting Chika but Escudo and Chain stopped Viza and Hyuse from chasing them any further. Yūma quickly separates Viza from Hyuse and engages in combat with him 1-on-1.

The battle between Hyuse and Jin turns to a tunnel as Jin was tagged by Hyuse's trigger. Jin uses this opportunity to speak up about how Aftokrator will abandon him on earth. The angered neighbor rushes him down, becoming trapped by Jin's Escudo blockades, putting the battle at a temporary stalemate. During the battle between Viza and Yūma, Viza severs Kuga's arm separating Replica from his body. After learning of Viza being a distraction, Kuga tells Replica to protect Osamu and Chika as Replica quickly flies to the two.

Soon enough, Izumi, Midorikawa and Yoneya unite with Karasuma and Osamu, battling with the Rabbits and helping the C-Rank agents escape. Osamu was pulled down by Hyuse's trigger in front of one of the Rabbits, but Chika arrives to lend Osamu her Trion to help destroy the Rabbit Izumi was battling earlier, along with the golden rabbit that targeted Osamu. Izumi suggests that he and Tamakoma Second finish off the soldiers but Chika senses Hairein's presence, who quickly activates Alektor. Izumi tells Osamu and Chika to escape and leave Hairein to himself. Osamu and Chika retreat, but Mikumo sees Izumi pinned and decides to return with Chika to save him using her trion to stop the trion bullets. The attempt failed, and the bullets struck Chika turning her body into a large trion cube. A distraught and demoralized Mikumo is instructed by Izumi and Karasuma to retreat as fast as possible. Osamu is quickly attacked by two rabbits as he was nearing the base, soon finding himself pinned down by the two soldiers, only to be saved Replica's shield. Replica uses Gate and forms a Rabbit copy to help Osamu as he moves closer and closer to the base.

In the tunnel, Jin informs Hyuse of Enedra's death at the hands of Mira but Hyuse does not believe him and continues his onslaught.

After defeating Karasuma and Izumi, Hairein approaches Osamu and Replica who hides in buildings as a chase ensues. Mikumo gets struck by a jellyfish bullet and has trouble moving, and soon becomes cornered by the neighbor, only to be saved at the last minute by Shūji Miwa. He advises for Miwa to transport Chika to the base, but Miwa kicks him aside to confront Hairein, giving Mikumo the chance to escape. Before the escape however, Osamu and Replica launch one last asteroid shot before leaving to deal with Mira. Replica sends in his Rabbit copy but is cut in half by Mira's Trigger leaving Osamu in a blind panic, inching closer towards the base. Replica quickly switches over to his backup system, removing the tags from Osamu's arm, which gives him an idea. Meanwhile, Kuga continues to battle Viza, using Bound to charge forward, slipping past Organon. Viza severed Kuga in half as he approached, believing that he won the battle, but as Kuga charged forward, he activates boost one last time in his second trion body, punching Viza into two. With that, he rushes as fast as possible to Osamu and Replica.

Osamu makes his move towards the base, with Hairein and Mira attacking Osamu and forcing him to deactivate his trion body. Yūma approaches, covering Osamu from above with Bolt Quinte. As Mikumo inches nearer, he is impaled by Mira's trigger. Hairein was distracted by the snipers, Yūma, and Miwa using Fūjin. The critically injured Osamu launched Replica into the gate to reprogram the neighbor ship to auto pilot. Replica bidded farewell to the fallen Mikumo and tells him to take care of Yūma for him. With no choice, The Aftorkrator soldiers retreated, abandoning Hyuse in the process.

In the aftermath, Yoneya and Izuho rush in to get Mikumo to the medical facility but demands that the two search for cubified Chika as he is wheeled away by the medical staff. Izuho and Yoneya search for her cube body but are attacked by two marmods that Yoneya rapidly destroys, with Kuga saving Chika from destruction. Jin's premonition of Hyuse being abandoned was true; and tells him to surrender to which he accepts.

After the Invasion
Osamu went into a week-long coma that delve into his past memories from his Border entrance exams and Rinji's disappearenceand woke up to see his Mother, informing him that a multitude of people from Border came to check on him as well as Usami, who announced the awarded agents; including Kuga, who earned the Distinguished Combat Award, and Mikumo himself who earned the First Class Distinguished Combat Award. Kuga finally arrives to see Osamu, informing him that Replica is possibly still alive, adding another incentive for the Squad to reach A-Rank. Much later, Foreign Affairs Manager Katsumi Karasawa escorts the trio alongside Kasumi to a press conference where Eizō Netsuki planned to shift the blame for the casualties of Mikado City onto Osamu, only for Mikumo to appear on stage and reveal the intentions of the away mission, assuring to the press that Border will rescue all of the missing persons from the Invasions.

B-Rank Rank Battles Arc

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

Trivia

 * Excluding Usami, all the original members' surnames have something to do with the sky. Kumo from Mikumo means "cloud", ku from Kuga can mean "sky" or be combined with 気 to mean "air", and ama from Amatori means "rain" if read in the appropriate context. Ama means "dive" by itself.