Hanamaru-sensei is a manga artist professionally collaborating with Yamamoto Mayumi, a part-time assistant at a manga publisher. During their work, he develops romantic feelings for her. Mayumi remains largely oblivious to his affections, echoing her sister Nanako's similar unawareness in romantic contexts. This dynamic continues through much of the narrative, as Hanamaru-sensei's attempts to express interest are consistently misunderstood or overlooked.

His pursuit culminates in a direct confession to Mayumi in chapter 92. Despite her typical obliviousness, this leads to significant relationship development. By the story's conclusion, Hanamaru-sensei and Mayumi are an official couple. They become engaged, resolving his romantic arc and establishing them as one of the series' confirmed endgame pairings.

The character's background beyond his profession and relationship with Mayumi remains unexplored. His role primarily explores themes of unrequited affection and eventual mutual understanding within the ensemble cast.