Maeng Sunjeong, more frequently known by her nickname Busandaek or Osaka-san, is a transfer student who joins the main friend group at the start of their high school years. Her real name is largely forgotten by her peers and even her teachers soon after her arrival, with the nickname given to her by her classmate Tomo Takino sticking permanently due to her passive and unresisting nature. The nickname is a reference to her having previously lived in Osaka, though her actual background is more complicated, as she was born in Wakayama and lived in Kobe during elementary school. Despite only living in Osaka for a relatively short time, the association with the city defines her identity throughout the series.

Her personality is marked by a slow, dreamy, and deeply unconventional way of thinking that sets her apart from the other characters. Rather than being traditionally unintelligent, her mind operates in a unique, often abstract manner, leading to frequent daydreams, non-sequiturs, and bizarre observations about everyday life. This tendency is so pronounced that she is considered the primary source of the series' surreal and offbeat humor. For instance, she might imagine a friend's pigtails as detachable wings or propose an unusually morbid twist in a scary story where she herself is the hidden murderer. Her speech is unhurried and her manner is calm, which contrasts sharply with the more energetic and loud personalities of some of her friends. She finds amusement in simple or odd things that others might overlook, such as the act of retrieving a piece of candy from a pile of flour during a sports festival obstacle course.

Within the story, Busandaek occupies the role of the quintessential airhead, often providing comic relief through her strange logic and inability to focus in class. Her academic performance is poor, largely due to her habit of falling asleep or drifting off into her own world during lessons, much to the frustration of her homeroom teacher, Yukari Tanizaki. Physically, she is notably uncoordinated and inflexible, being the worst in her class at sports and unable to perform simple stretches without difficulty. Despite these shortcomings, she occasionally demonstrates surprising flashes of insight, being able to solve certain types of lateral thinking puzzles or recall obscure facts, such as details about marine biology, that others cannot.

Her relationships with the other main characters are central to her development. She forms a particularly strong and gentle bond with Chiyo Mihama, the ten-year-old child prodigy in the class. Despite their vast differences in intellect and maturity, they share a childlike sense of wonder and a mutual lack of athletic ability, and Busandaek often joins Chiyo for quiet or contemplative moments. She is also grouped with the energetic Tomo and the sporty Kagura as one of the knuckleheads, a trio known for earning the lowest grades and for their general lack of common sense. In this group, her calm spaciness acts as a foil to Tomo's relentless hyperactivity. Her quiet demeanor means she rarely initiates conflict, but she is capable of pointing out obvious truths that everyone else misses, such as the fact that none of her friends have romantic relationships.

Over the course of the three years of high school, Busandaek shows little traditional character development, remaining consistently true to her own unique nature. Her acceptance of her nickname and her place within the group becomes a defining trait, illustrating her inability to muster the energy for resistance. One of the rare glimpses into her future reveals that she eventually becomes a physical education teacher, a career that is highly ironic given her own poor physical performance and unusual thought processes as a student. Her enduring popularity among fans stems from her status as a pioneer of the endearing, four-dimensional character archetype and the memorable quality of her strange, gentle perspective on the world.