Daigorō Iwata is a shy, chubby elementary school student sharing a class with Chiyoko Kurotori, characterized by his black hair and frequent depiction with closed eyes in official art. His classmates originally honored him with the nickname "Yokozuna" to celebrate his exceptional sumo wrestling skills, which secured him a school tournament trophy—a title reflecting genuine respect for his athletic talent.

This changed when his teacher, Mr. Matsuoka, observed him taking three lunch portions and mockingly dubbed him "Kuishinbo" (glutton). The derogatory nickname swiftly replaced "Yokozuna" among students, causing Daigorō deep distress. He internalizes this unhappiness, seldom voicing his feelings, yet maintains an outwardly cheerful and simple-minded demeanor.

Mr. Matsuoka later apologized for his insensitivity and instructed the class to revert to "Yokozuna," emphasizing the importance of valuing Daigorō’s sumo achievements over the hurtful label. This episode illustrates how thoughtless teasing impacted his emotional well-being.