He is a young man who was injured in the same train crash like Otonashi. Igarashi befriends Otonashi and the two work hard to save the people in the train wreck.

Spoiler

When Otonashi decided to donate his organs, he motivated Igarashi to do so too, and soon everybody underground did that. His words of encouragement made Otonashi realize that he was somebody who helped others in his real life.