Igarashi is a student from an unknown institution who was among the many casualties in the train crash where Yuzuru Otonashi was also involved in. After surviving the collision, Igarashi rescued as many of the injured passengers as he can. He later meets Yuzuru, and they became one of the very few people who tries to organize the resources they need for survival after the accident blocked all possible exits.
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.
What happened to Igarashi after being rescued was not stated.