Motoharu Igarashi is a twenty-seven-year-old novelist and one of the main characters in the series. He joins the cast in the second half of the story, having first appeared in the original manga. He is described as a genius-type self-contained cool-headed man, meaning he possesses an innate composure and intellectual confidence that make him appear effortlessly collected. However, like the other characters, he is also prone to clumsy mistakes, which he handles with a relaxed, almost philosophical attitude, treating each blunder as a valuable learning experience rather than a source of embarrassment.

His background includes a childhood event that shaped his professional philosophy: he believes that one cannot write about people without truly knowing them. This conviction drives him to be exceptionally sociable and approachable, and he is able to form friendships with almost anyone he meets. His easygoing charm and natural ability to connect with others make him a skilled communicator, though he often seems unaware of the effect he has on people.

In the story, Motoharu serves as an older, experienced figure among the group. He is acquainted with Takayuki Mima, having known him since elementary school, and their history adds depth to his relationships. His role is to bring a more mature perspective to the group dynamic, while still participating in the same kind of endearingly awkward situations that define the series. Over time, Motoharu shows little need for drastic development; instead, his steady, self-assured nature contrasts with the more visibly flustered reactions of the younger characters, providing a calm anchor. His notable abilities include his talent as a novelist, his high emotional intelligence, and his ability to turn any social encounter into a meaningful interaction.