Motoharu Igarashi is a 27-year-old novelist defined by a cool-headed, "genius-type self-contained" nature. His writing philosophy, deeply rooted in childhood experiences, insists that "you can't write about people unless you know them," driving his observations and interactions. He forms friendships easily through strong communication skills.

His background includes attending Hayama Elementary School, where he met childhood friend Takayuki Mima. Their connection broke when Motoharu transferred schools, an event leaving a lasting impact and making their eventual reunion a significant focus. He frequently thinks about Mima, even during routine activities.

As an adult, he reconnects with Takayuki Mima, addressing him by his given name. During a park encounter where Takayuki is present, Motoharu also meets Hayate Ichikura, Shun Futami, and Souma Shiki, addressing each by their given names too, indicating familiarity.

Professionally, his novel centers on confessing mistakes and embracing imperfections, becoming a narrative device connecting the main group. Takayuki Mima handles publicity for the book's adaptation, intertwining their professional and personal lives.

His name combines the kanji "元" (origin/beginning) and "晴" (clear weather) for "Motoharu," while his surname "Igarashi" uses "五十" (fifty) and "嵐" (storm). This contrast parallels his composed exterior and the thematic clumsiness shared among the characters.

Within broader storylines, he integrates into the group dynamic, contributing to scenarios navigating social awkwardness and mutual support. His presence bridges younger and older characters, reinforcing the series' focus on everyday connections and personal growth.