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.