On the surface, he appears to have an outgoing and sociable personality. However, this was revealed to be an act to hide his anger. He also appears to be a drug dealer secretly and goes by the codename "Sugar". Surprisingly, on the inside, he views his infected brother Jun as a burden to him, saying he was always in the way.
He is first coming with Souta to ask Shu about a video they were making for a club project. Yahiro tells Souta to not rush Shu and let him get it done by the deadline and they both leave. He is seen again when Inori appears as their new classmate and tells everyone to leave her alone, since she'll be with them for the rest of the year. Later, he comes by Shu's house since he thought he was acting weird and decided they should watch a movie together. Yahiro is shocked to see that Inori was at Shu's house and asks him what circumstances led to her staying with him. When Shu says he wasn't sure, Yahiro asks Shu to talk to him if anything were to happen. The next day at school, Yahiro hides Inori and Shu from an angry Kanon (because Shu gropped her accidently). Yahiro is later revealed to be the person Gai was looking for who witnessed the incident that happened with Daryl. After pulling out Yahiro's Void and saving his life by preventing Inori from shooting him, Shu and Yahiro make a promise: That Shu wouldn't say anything about the Norma Gene and Yahiro wouldn't say anything about Shu's involvment with the Undertaker. However, this promise is broken when Yahiro pushes Shu out of a train after a short stop, right into the hands of Major Segai and his men. He says he's not going to appologize for anything. He is later seen visiting his younger brother Jun, who still suffers from the Apocalypse Virus. It is revealed that he sold out Shu to help his brother and had been hiding in a church until the GHQ finds them. However, they escape and Yahiro eventually meets up with Shu when he runs into the same train he was in. They talk alone, in which he reveals that the GHQ were going to put Jun down (during the events of episode 4) and they used the confusion as a way to escape. Later, they go to their new hideout where Shu meets Jun for the second time. Shu tells Yahiro that he'll help him, to which Yahiro is shocked at. However, Shu tells him to not get the wrong idea, since he was just helping Jun, who didn't do anything wrong. They are attacked by the GHQ's Anti Bodies in between which
Shu and Yahiro have an argument. Yahiro questions if Shu was the one who sent the GHQ there but Shu yells at him to shut up, calling him a traitor (which Yahiro is shocked at). Shu further shouts that he should just shut up and let him "use" him. After falling unconsious from having his Void removed, he awakens to find out that his brother, Jun, was dead.