Sunao Moriyama is a serious and emotionally reserved junior high school student. He lives independently for long stretches due to his father Kōdai's frequent work travel, a situation arising after his mother's death forced early self-sufficiency. His domestic life is simple, often relying on basic foods like bread.

One morning, he discovers a small, childlike creature in his refrigerator. Inspired by the potato and mayonnaise bun he's eating, he names her Potemayo. He assumes responsibility for her care, taking her to school daily where she typically rides on his head. He is attentive to her needs and safety, though Potemayo's intense jealousy towards classmates who approach him creates complications.

His relationship with his father Kōdai is strained. Kōdai, a folklorist with a playful and immature personality, sporadically visits home. Sunao views his father's behavior with resentment and disdain, often responding to his antics with physical aggression like punching; this resentment stems partly from his father's absence following his mother's death. Kōdai whimsically refers to Potemayo as "Mochi-Mochi."

At school, Sunao's stoicism defines his interactions. He navigates Potemayo's disruptive presence and the arrival of a second, more destructive creature named Guchuko, largely maintaining his composure. A running gag involves his severe nearsightedness without his glasses. His classmate Natsu Mikan harbors a significant crush on him, leading to frequent daydreams and attempts to get closer, which Potemayo aggressively disrupts. Sunao remains largely passive amidst this rivalry.

While generally emotionally reserved, subtle development occurs later. Episodes exploring his mother's death and his father's efforts to connect prompt moments of reflection and a slight softening in his rigid demeanor. His deceased mother, Miku, remains an offscreen influence, having imparted values of kindness before her illness.