Ayumu Kasuga, known almost exclusively by her nickname Osaka, is a main character known for her profoundly absent-minded and dreamy nature. Her real name is rarely used, as her nickname was given by her friend Tomo Takino shortly after she transferred into their high school class. The name is a reference to her previous residence in the Osaka region of Japan, though the choice is ironic because her personality is the opposite of the loud and gregarious stereotype associated with people from that area.

Physically, Osaka is depicted as being of average height, standing around 156 centimeters tall, with a slender build and large, expressive eyes that often appear unfocused, reflecting her inner world. Her most defining trait is her unusually slow and idiosyncratic way of thinking. She is often lost in vivid daydreams, which frequently feature her friend Chiyo Mihama's distinctive pigtails in surreal scenarios, such as being detachable appendages that allow Chiyo to fly or being independent, intelligent creatures. This tendency to mentally drift away often leads to her falling asleep in class, much to the frustration of her teacher, Yukari Tanizaki.

Osaka is not motivated by typical goals like academic success or social popularity. Her actions are driven by a quiet, childlike curiosity about the world and a desire to understand small, often overlooked details. This manifests in a fascination with unusual topics, such as sea slugs, and a penchant for bizarre and sometimes morbid humor. For example, she once proposed a scary story where she herself murders all her friends, and as a compromise for a festival theme, she suggested a cafe decorated with cat corpses. Despite her slow demeanor, her mind works in a unique way that makes her unexpectedly adept at solving certain types of riddles and lateral thinking puzzles, much to the astonishment of the academic prodigy Chiyo.

Her role in the story is primarily as a source of surreal comedy and a catalyst for strange situations. She is a core member of the group of friends, and her relationships are defined by how the others react to her eccentricities. She shares a particularly close bond with Chiyo, who often acts as a gentle anchor to reality, physically grabbing Osaka's sleeve to prevent her from acting on a dangerous or nonsensical impulse. Osaka also has a notable dynamic with Tomo, whose loud and energetic personality is a direct foil to her own placid stillness, and with the athletic Kagura, as both are part of the informal group of academically struggling students known as the "Bonkurazu". Her calm and quiet mood is rarely broken, but on the rare occasions it is, such as when she finally masters the technique of breaking chopsticks evenly, her reactions are notably intense.

Throughout the three years of high school, Osaka shows little conventional development in terms of becoming more focused or academically diligent. Instead, her character deepens as the friends come to understand and accept her peculiar logic. A significant moment occurs near the end of their school life when Chiyo, unaware of the implications, suggests that Osaka become a teacher. The suggestion is taken as a profound insult by their actual teacher, Yukari, highlighting how others perceive Osaka's unique worldview as incompatible with traditional responsibilities. Her physical abilities are consistently portrayed as poor; she is notably inflexible and was the first person in the group to lose a footrace against the much younger and smaller Chiyo. However, her true abilities lie not in practical skills but in her singular, imaginative perspective that turns mundane high school life into a series of strange and memorable events.