Yūichi Jin/Plot

Background
At some point in his past, his mother was killed by a Neighbor. He was a member of Border before it went public. Before the start of the series he saved Osamu from a Bamster when the latter attempted to sneak into Border. He went on several missions to the Neighborhood, getting to know that Neighbors weren't all bad.

Introduction Arc
Jin is first seen sitting upon slain Trion Warriors. He comes to Osamu when his punishment is being decided by the upper echelons. He defends Osamu, saving him from being dismissed from Border. Later, he meets Yūma, who has information regarding the Rad Infestation. He next appears during Yūma's battle with Miwa Unit. He persuades Narasaka to not shoot Yūma, and then comes at the end of the battle. He and Osamu are sent back to Border's echelons, this time Osamu is in trouble for hiding a Neighbor. Kido orders Jin to steal Yūma's Black Trigger.

Black Trigger Capture Arc
Jin refuses to obey, forcing Rindō to order him, though Rindō orders him to go about it as he wished. He meets with Yūma and asks him to join Border, stating that he would be safer. Yūma goes to Border only to find out about his father's acquaintance, Mogami, only to find out that he passed away 5 years before. He was also Jin's mentor, and the one who became Fūjin through death. Hearing about this, Yūma considers returning to the Neighborhood, but Jin persuades him not to, stating that it would be a lot of fun to be on this side. Jin's words help; Yūma decides to join. However, as Border Official Enlistment day did not occur, he was not an official member of Border meaning that Border could hunt him down. Kido uses this to his advantage by sending Border's top three units after him. They, along with Miwa Unit, are intercepted by Jin and Arashiyama Unit and get into a fight. Jin fights his rival, Tachikawa, and claims victory over him, and subsequently, over his foes. The conflict is still not over, as Kido threatens to send Amō. To ensure Yūma's safety, Jin offers Fūjin in exchange for Yūma's safety and guaranteed enlistment. The deal is successful, leading to Jin being demoted to A-Class and losing his Black Trigger, but at the same time, Yūma could now not worry about being pursued by Border.

Border Enlistment Arc
After Yūma, Chika, and Osamu make their flashy debuts on Enlistment day, Jin receives a call about it whilst defeating Bamsters. He says that there were more surprises to come. After Yūma's battle with Midorikawa, he summons Osamu and Yūma to the meeting regarding the predicted invasion. There, he listens as Replica explains about the Neighborhood. He reveals that he can't see whom the invaders will be since he has not met them. After the meeting, he goes to the rooftop to ask a favour from Miwa. He explains that in the upcoming invasion, Osamu would get himself into a predicament, at which point Miwa would have to save him. As payment, he offers to recommend Miwa for possession of Fūjin. Miwa refuses, but Jin assures him he would when the time came. When the invasion is about to begin, he notes that they were early, and prepares for battle.

Large-Scale Invasion Arc
He and Amō are sent to the west and northwest to fight the Trion Warriors there. When Amō is done with his part, Jin has him take over his own. He uses the opportunity to reunite with Yūma. He tells him of the worst future: Osamu's death. To prevent it, he and Yūma had to fight the invaders. He reveals that he got permission from Kido to have Yūma fight at the edge of the Danger Zone. He and Yūma travel there. When Viza and Hyuse pursue Chika, he and Yūma intercept them. Since he and Yūma were unfamiliar with each other's abilities, they decide to fight separately. Jin fights Hyuse and lures him into an underground tunnel. While Hyuse seemingly has the advantage in the darkness, Jin taunts him by revealing that he would be abandoned. Enraged, Hyuse charges at him, but Jin traps him with Escudo. Hyuse manages to break out of the shield and they keep fighting until Hairein takes off without him, effectively marooning him on Earth. A car drives up and Jin recommends that Hyuse turns himself in, lest suffer a worse fate.