Rinji was Osamu's tutor. Months before the start of the story, he told him Chika's story and her friend's disappearance. He told Osamu that, if something was to happen to him, he should protect Chika, which was the reason why Osamu joined Border. On April 28, he revealed his mission to try and go to the Neighborhood, and on May 2, he was declared to be missing. He was suspected to have collaborated with Mirai Hatohara, who had stolen Triggers from Border.
After Osamu defeats the Vander, saving Yūma and Chika, and takes them to Yumide City Station, Osamu tells Yūma about Chika's story and his reason to join Border.
At the shrine, Chika asks Yūma what happens to humans taken to the Neighbor World. After he explains, he asks why she wants to know. At first, she dismisses his question by saying she was simply curious, but after Yūma points out to her he was aware she was lying, she apologizes to him. She then tells him about Rinji and her friend, and why doesn't she want to be involved with anyone.
After talking to Usami about the Neighbor World, she asks to join Tamakoma Branch, in order to go after her brother and friend herself.
Chika remembers Rinji as a reason she wanted to join Border so that she can gather the mental strength to fight a Rabbit. Later, when Osamu is unconscious, he reminisces the days leading to Rinji's disappearance. It is revealed that he had not been kidnapped by Neighbors, but rather willingly opened a gate and went to the Neighborhood. He had collaborators who had supplied him with Triggers from Border and we're going to try going to the Neighborhood independently. When Osamu tried to tag along, he told him that it was too dangerous. After he did not return, authorities were contacted, and Osamu receives a pre-written email from him stating that if he didn't return, Osamu would receive the email, and asks him to take care of Chika.
Ninomiya discusses the topic of Hatohara's disappearance and brings up the possible connection to Rinji's disappearance. He believes that Rinji had initiated her to steal Border Triggers and then enter the Neighborhood. While Chika believes that he could, Osamu defends him, saying that Rinji had mentioned collaborators.