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 Yōsuke 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 big Neighbour, and asked which unit was responsible with defeating it. Yōsuke assumed it was the doing of an A-rank. Miwa was then informed that no other unit was sent to the area, meaning they were the first unit to get there. This confused him, causing him to wonder who was the responsible.

He was then seen again in Chapter 10, when Osamu was 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 was quietly watching the reunion.

Jin then arrived and Kido thanked him for coming, and then introduced himself to Osamu, as if they didn't know each other yet. Kido started the reunion by stating they should do something about the irregular portals, but Shinoda asked him to wait, saying they first should reach a consensus regarding Osamu's punishment. Kido answered saying it was already decided (he'd be fired).

Shinoda defended that Osamu should not be punished, and should instead be promoted to B-rank, to which Mr. Kinuta and Mr. Netsuki contested. Kido then said that he had no place in his organization for people who could not follow a single rule, and fired Osamu (thought he was later reemployed with the help of Yūma and Jin).

After the reunion was over, Miwa asked Kido's permission to follow Osamu, which he allowed him to.

Black Trigger Capture Arc
On Kido's orders, Miwa Unit was 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.