Igarashi is a student from an unknown institution who was among the many partaking in the deadly train crash where Yuzuru Otonashi was also involved in. After surviving the collision, he staggers along to meet Yuzuru in his train car. Yuzuru helped him off the train. He gathers enough strength to rescue as many people off the train as he can. The two of them became the supposed leaders of the very few people who survived. Yuzuru discovered both sides of the tunnel blocked off. They were trapped in a cave.
With his and Yuzuru's leadership skills, most of the rescued survivors were able to survive seven days with only water and a small amount of food as a source of nourishment. However, Yuzuru wasn't able to make it as his stomach wound gravely worsened his condition, causing his death at the very moment rescuers arrive. Igarashi is heard yelling to Yuzuru in his last moments as they became close in the seven days trapped together.
What happened to Igarashi after being rescued was not stated.