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, Yuma and Chika were invited to Tamakoma branch by Yuichi Jin, after asking Yuma 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. Yuma and Osamu are then called to meet with Rindo. After hearing that Soichi Mogami, the person that Yuma was trying to meet had died, Rindo asks Yuma to join border to which Yuma denies.

Moments later, Replica explains Yuma's reasons to coming to earth: meeting with Mogami Souchi in an attempt to revive his father. He then tells Osamu about Yuma nearly dying and his father sacrificing his life to save him at the last minute. Yuma 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 and her friend Aoba. Though indecisive at first about her decision, the two accept her decision as Osamu then heads up to Yuma. During the conversation, Osamu asks Yuma to join in hopes to help bring back Chika's comrades then asks for his assistance. The two talk about Osamu's selflessness, then Yuma 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 Yuma, He simply explains that Osamu being captain is the way that it should be. After hearing the good news, Rindo assigns Osamu, Yuma and Chika to being a squad.

Black Trigger Capture Arc
The next morning, Shiori and Jin give Osamu, Yuma and Chika the rundown on the Border enlistment and its ranking system. With Yuma 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 Yuma's disappointment, Jin also explains that Yuma could not use his Black Trigger due to the Triggers being considered S-Rank Triggers and prohibited from mock battles. Shiori then asks Chika what position is best for her to which Yuma answers to her being a combatant owing her to her massive trion levels. Shiori began to explain the team positions based on their combat ranges and 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 began to determine the best role for Chika, Jin interrupts her using his side effect and says 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 Kyosuke 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 Yuma, Shiori suggesting Chika to be taught by Reiji and Karasuma picking Osamu by default.

Later on, Shiori and teach 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 Yuma and Konami use the next rooms to start there respective training. During the training, Konami defeats Yuma multiple times with Yuma 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, Yuma 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 expidition 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, Yuma and Chika split into their respective groups Arashiyama explains the ranking system and how promotion works, giving Yuma much more determination than before.

During the simulation training, Yuma sets a record in the Bamster training by finishing the exam in less than 0.6 seconds then when asked to do it again finished 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, Soya Kazama challenges Osamu to mock battles to which Osamu accepts without hesitation, and loses 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 Yuma'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 Yuma'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 Yuma 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. Yuma 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 Yuma to see commander Masamune Kido.

In the meeting, Yuma, 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 Yuma's safety so long as he is with Border to which Kido complies. Meanwhile, Chika and Izuho continue their training with Izuho envying Chika's progress to which Chika responds with helping Izuho become a better shiper. After the meeting, Shinoda communicates with Yuma and Osamu with much praise with the information given and suggests that Yuma 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.

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".