Shūji Miwa/Plot

Background
Miwa hates all Neighbors because they killed his older sister. He joined Border during its debut to avenge her death.

Introduction Arc
Miwa and Yoneya are first seen in the beginning of Chapter 2, investigating the Danger Zone after Yūma Kuga's battle with the Bamster. He contacted HQ, confirming the destruction was caused by a large Neighbor, and asked which unit was responsible for defeating it. Yoneya assumes it was the doing of an A-rank. Miwa is then informed that no other unit was sent to the area, meaning they were the first unit to get there. This confuses him, causing him to wonder who was responsible.

He is then seen again in Chapter 10, when Osamu is called to the HQ by Kido to decide his punishment regarding the incident in Osamu's school, as he broke Border's rules by using his Trigger (which was developed for training only) to battle, despite C-rank members not being allowed to battle. He is shown quietly watching the reunion.

Jin then arrives, with Kido thanking him for coming, and then introduces himself to Osamu as if they had not met before. Kido starts the reunion by stating they should do something about the irregular portals, but Shinoda asks him to wait, saying they should first reach a consensus regarding Osamu's punishment. Kido answers saying it has already been decided that he'll be fired.

Shinoda defends Osamu, claiming that he should not be punished and should instead be promoted to B-rank, to which Mr. Kinuta and Mr. Netsuki protest. Kido then says that he has no place in his organization for people who can not follow a single rule. Jin then interjects, convincing the heads of border to leave the decision to him, which would later lead to Osamu being promoted to B-rank.

After the reunion is over, Miwa asks Kido's permission to follow Osamu, which he allows him to do.

Black Trigger Capture Arc
On Kido's orders, Miwa Unit is sent along with the Expedition Force to secure Yūma's Trigger. When they are intercepted by Jin and Arashiyama Unit, he joins with Izumi to fight Arashiyama and Tokieda. He uses Lead Bullet to incapacitate Arashiyama, but ultimately it is to no avail as Jin and Arashiyama Unit win the battle. Miwa complains that Jin does not know what it is like to lose one dear to him, and is too trusting of Neighbors, but Arashiyama corrects him by revealing that Jin's mother was killed by a Neighbor, and that his mentor died a few years prior.

Border Enlistment Arc
Miwa appears before Yūma. He is clearly upset over the fact that Yūma was enlisted into Border, so Yūma tries to settle with him by offering to help locate the country responsible for his sister's death, much to Miwa's confusion and surprise. Miwa angrily dismisses him, declaring that all Neighbors are enemies. He goes off to a meeting. Later, he is seen at the meeting concerning the Second Large-Scale Invasion. Following the meeting, he sits on Border's roof. Jin meets him there, revealing he has an important piece of information. He explains that in the upcoming invasion, Osamu would find himself in an ordeal, and Miwa would be the one to save him. He uses the fact that Miwa was a candidate for Fūjin to entice him. Miwa refuses to help, but Jin assures him that he would do it when the time came.

Large-Scale Invasion Arc
He is first seen defeating Rabbits. Later, when Osamu is trapped by Hairein, he appears, now realizing what Jin meant. He then gets into a battle with Hairein, where his Lead Bullet and Replica's support proves to be an effective countermeasure against Alektor. Hairein and Mira attempt to strike with Alektor bullets through a portal behind him, but he had already heard of the tactic being used against Karasuma, and shoots the portal, severely wounding the two. Mira teleports him away, so he uses Fūjin to strike Hairein, which ends up helping Osamu.