Yūma Kuga 4

Short summary
Yūma tells Jin about his past, while Replica tells the same story to Osamu. Later, Yūma tells Jin about his decision to go back to the world on the other side, and Replica asks Osamu to give Yūma a new goal.

Long summary
Replica tells Osamu that four years ago, the Kuga's joined the wars in their world as a favor to Raymond, the leader of their country's defense and Yūgo's long-time friend. Yūma tells Jin that he was well-trained and did well as a rookie, but Replica explains everything changed on the day Yūgo died.

Replica explains to Osamu that four years ago, after their fortress was attacked, Raymond and Yūgo told Yūma to stay out of the battle, since the attackers most likely had a Black Trigger. The enemy's forces suddenly attacked, and the Calvarian Trigger controllers were outnumbered. Yūma decided to join the battle, disregarding his father's orders. A strange Neighbor with claws appeared and attacked Yūma, leaving him fatally wounded. In order to save his life, Yūgo created a Black Trigger, sealing off Yūma's dying body inside and replacing it with a Trion body, turning his hair white. When Yūma woke up, he watched his father turned to dust, crumbles and dies.

As Yūma mourned for his father, the military pressured him into using the Black Trigger to seek vengeance for Yūgo's death, not knowing that Yūma also inherited his father's Side Effect, enabling him to see through lies. Regardless, Yūma decided to finish the fight the two of them started and fought in the battle for three years, getting stronger in the process, until the war ended in reconciliation. After the war, Replica suggested that Yūma find Border and his father's friend, hoping that someone there could help Yūma get his old body back.

Replica explains to Osamu that Yūma has not grown since he was 11 years old, because he has a body made out of Trion. He adds that inside the Black Trigger, Yūma's original body is still slowly dying, and once it completely dies, Yūma's Trion body will also die. While Replica wanted someone to help Yūma gain his original body, Yūma actually hoped to find a way to resurrect his father. After hearing the news about Mogami, Replica knows that Yūma no longer has any plan for his life. At the same time, Yūma informs Jin that he have decided to go back to the other side. Replica then implores Osamu to help Yūma find a new purpose.