Yūma Kuga/Plot

Background
When he was 6, Yūma's father Yūgo Kuga taught him "The Three Teachings:" "protect your own hide," "there's more than one right answer," and "parents aren't always right."

When Yūma still lived in the Neighbor world, he and his father, Yūgo, participated in the Civil War between Calvaria and Spinthir, helping Calvaria as a favor to a friend of his father. One day, Yūgo told Yūma not to participate in the battle because earlier that day they had found the dead bodies of some of their men, and they were some of the most powerful of their side. Yūma's father suspected it was the result of the other side having a Black Trigger, so he wanted to keep Yūma out of danger. Yūma, however, just pouted and didn't really think much of his father's words.

After his father had left to go fight, Yūma left Calvaria's fortress and went to the battlefield. Seeing multiple battles he could help win, he decided to make attacks behind the scenes and eliminate some of the enemy units. But before he could do that, he was approached by a mysterious enemy and wasn't strong enough to defend himself, resulting in his death. As Yūma laid there dying, his father came to his aid and took out his own trigger. He then created a black trigger which sealed the body of the dying Yūma inside the trigger, substituting it with a trion body (which is the reason his hair is white); thus saving Yūma. Unfortunately, this took all of Yūgo's power. And so he turned to dust, crumbled and died. However, this does not mean Yūma is immortal; Yūma's real body is sealed inside his ring and is slowly degrading. And once his body inside his ring is fully gone, it is most likely Yūma's trion body will also be terminated.

Introduction Arc
Yūma is first seen after getting involved in a car accident. Despite being hit by a car, he is unscathed. He rushes to school, late, since he investigated Border HQ first. Arriving at school, Osamu is assigned as his partner. He is picked on by the 3 Idiots but proceeds to overwhelm them. After school, the bullies retaliate, forcing Osamu and Yūma into the Danger Zone. A Bamster appears and begins to eat one of the Idiots. Osamu rushes in to save him but is easily overpowered by it. Yūma decides to use his Trigger to fight the Bamster and defeats it easily. After being asked about it, Yūma reveals that he's not part of Border, but rather, a Neighbor. He and Osamu flee the scene to avoid being found by Border. As they wander throughout the city, Yūma does things, such as taking out a large amount of money in public, to attract negative attention. He proceeds to thrash them, much to Osamu's horror. Yūma explains that this was the norm for him in the Neighborhood.

The next day, he fends off the 3 Idiots without actually hurting them, which earns him the attention of his peers. When they ask him about his background, he states that he was from a place with a lot of war. It is here that he learns of the First Large-Scale Invasion, and people's hostility towards Neighbors. He nearly reveals that Osamu is a Border member. Osamu takes him to another room and orders him to not say it. He also threatens to report Yūma to Border if he commits a crime. Yūma notes he was lucky to have Osamu as the first Border agent he met. Just then, an Irregular Gate appears beside the school, releasing Marmods. Osamu is insistent on fighting them and protecting the students. Yūma warns him that it would be a certain loss, leading to death, but Osamu goes in anyway. When Osamu is about to be killed, Yūma comes in and uses Osamu's Trigger to defeat the Marmod. He lies to everyone that Osamu destroyed the Marmod to avoid being found out. When Arashiyama Squad comes, he defends Osamu against Kitora's accusations, comparing the situation to whether Border gets authorization from Neighbors to use Triggers. Using his Side Effect, he accuses her of being jealous of Osamu. He is very surprised that Osamu's heroic actions led only to punishment.

He accompanies Osamu as he is escorted to HQ by Kitora. He gets into more arguments with Kitora. When a Ilgar attacks, he uses his Black Trigger to support Kitora and bring it down. She, however, is not aware of this.

He goes to school and is investigating the cause of the Irregular Gates at night. The next day, he reveals he has pinpointed the cause: Rads, Trion Warriors capable of opening gates. Jin appears and reveals himself, though he promises he would not reveal Yūma's existence to Border. Instead, he is quite curious about Yūma.

Near Yumite District Station, he tries to ride a bicycle but crashes into the water, horrifying Chika, who has, unbeknownst to Yūma, been summoned by Osamu as well. When the Chika starts running into the Danger Zone, he follows her. When she is attacked by the Bander, he saves her from harm. After Osamu defeats it, he asks Yūma to find a reason Chika seems to attract Trion Warriors. Yūma has Replica measure her trion, which is unduly large. Miwa Squad, having watched the situation, comes to arrest Chika, believing she's the Neighbor. Yūma reveals it's him and fights Miwa Unit. Despite being initially overwhelmed, he uses his Trigger to copy and strengthen Lead Bullet to defeat his opponents. The outcome of the battle, however, reveals his existence to Border. Kido orders Jin to capture his Black Trigger.

Introduction Arc

 * Yūma Kuga vs. Miwa Squad: Win

Border Enlistment Arc

 * Yūma Kuga vs. Shun Midorikawa: Win (8-2)

Large-Scale Invasion Arc

 * Yūma Kuga vs. Viza: Win

B-Rank Wars Arc

 * Tamakoma Second vs. Yoshizato Squad vs. Mamiya Squad: Win
 * Tamakoma Second vs. Suwa Squad vs. Arafune Squad: Win
 * Tamakoma Second vs. Suzunari First vs. Nasu Squad: Win
 * Tamakoma Second vs. Ninomiya Squad vs. Kageura Squad vs. Azuma Squad: Loss (Ninomiya Squad's victory)
 * Tamakoma Second vs. Katori Squad vs. Kakizaki Squad: Win
 * Tamakoma Second vs. Ōji Squad vs. Ikoma Squad: Win
 * Tamakoma Second vs. Kageura Squad vs. Azuma Squad vs. Suzunari First: Win
 * Tamakoma Second vs. Ninomiya Squad vs. Ikoma Squad vs. Yuba Squad: Win