Issei Miyata, often referred to by the nickname Gum or Gum Boy, is a secondary character in the story who shares a classroom with the two main protagonists. His nickname derives from his most recognizable habit: he frequently offers a stick of gum to the male protagonist, Haruki, an offer he repeats throughout the narrative. Physically, he is depicted as a male adolescent with blonde or yellow hair worn at ear length and brown eyes, typically seen in his school uniform.

In terms of personality, Gum is presented as a friendly, persistent, and ordinary individual. He is not defined by any dramatic personal struggles or extraordinary traits. Instead, his character embodies a simple, cheerful normalcy; he is described as a nice guy who enjoys everyday social interactions with his peers. His defining action is his consistent, good-natured attempt to connect with Haruki, despite the protagonist's initial aloof and unsociable behavior.

Within the narrative, Gum's primary role is functional rather than central. He serves as a subtle but effective barometer for Haruki's personal growth and emotional development. At the beginning of the story, Haruki is isolated and rejects social advances, systematically ignoring Gum's offers. As Haruki spends time with the terminally ill Sakura and begins to open up, a significant turning point is marked when he finally accepts the piece of gum. This simple act signifies his newfound willingness to engage with the world and form casual bonds with those around him. Through this quiet, recurring interaction, Gum helps illustrate one of the story's central themes: the small, often overlooked connections that make up daily life are profoundly meaningful.

Gum does not have a direct involvement in the central secret of Sakura's pancreatic illness; his role exists entirely within the ordinary sphere of high school life. His key relationships are therefore limited to being a friendly classmate. He has no notable specialized abilities or unique skills, as his importance to the story is derived entirely from his simplicity and persistence as a peer. His character remains largely static, but this consistency is what allows him to effectively highlight the transformation occurring in the protagonist.