Chiaki Minami is the youngest Minami sister, an elementary school student in class 5-2. Her name includes the kanji for "autumn," fitting her family's seasonal naming pattern. Highly intelligent for her age, she often reads thick textbooks, yet overanalyzes simple situations and lacks practical common sense, like believing water consumption improves swimming. Her personality mixes sharp cynicism beyond her years with moments of childish naivete. She frequently uses the insult "bakayarou," especially targeting her middle sister Kana and classmate Makoto, and later tells annoying visitors to "Go home!" She dislikes vegetables like carrots, peppers, and scallions, though her aversion to carrots may have lessened; she favors cola, curry, deep-fried foods, and sweets. She struggles with swimming and misinterprets mature topics as "Strange Things." She possesses a stuffed bear named "Fujioka," used to hide items or as a projectile, often hurled at Kana. She excels at board and video games, consistently defeating Kana. Her classmates call her "hime" (princess), a nickname she invented but quickly came to dislike. She is of average height for her age, with hazel, light brown, or red eyes depending on the adaptation, and long blonde or dark blonde hair featuring a distinctive leaf-shaped ahoge that twitches with her emotions. She is consistently depicted as the most flat-chested among her peers, finding this irritating when mentioned.

Her family background suggests parental absence, leading her to create a surrogate family. She views her eldest sister Haruka as a maternal figure, addressing her with extreme reverence as "Haruka-nee-sama" and using exclusively polite Japanese. This devotion motivates Chiaki to protect or impress Haruka, potentially influencing her academic efforts. Her relationship with Kana is outwardly antagonistic, marked by physical comedy like punching or throwing the bear, and frequent insults dismissing Kana's intelligence. Underlying affection exists, shown when Chiaki clings to Kana after dreaming of her disappearance and displays intense distress over a fake suicide note from Kana. Their dynamic includes mutual, though often misguided, support and genuine concern. After Haruka notes Fujioka resembles their father, Chiaki treats the bear as a surrogate father, sitting on his lap for affection and showing possessiveness, like jealousy when Touma interacts with him. She designates classmate Touma Minami, a tomboy from class 5-1 sharing her surname, as her surrogate "little brother," ignoring Touma's actual gender and justifying it with "winter follows autumn." Their friendship becomes very close, bordering on best friends by the "Okaeri" season. Chiaki also demonstrates concern for characters like Fuyuki, attempting to reform his passivity and showing visible sadness when he moves away.

At school, Chiaki holds unofficial authority over class 5-2, rarely exercised except over Makoto, whom she constantly torments and commands. Her primary social circle includes classmates Uchida and Yoshino, alongside Touma. Interactions with the school nurse, Kumada, provide comedic moments. Her catchphrase "bakayarou" and habit of assigning nicknames, often targeting weaknesses, are signature traits, though her self-applied "princess" moniker backfired. Despite her mature affect, she retains childlike tendencies like gaming prowess, vegetable dislike, and attachment to Fujioka. Key developmental moments include sabotaging Fujioka's confession to Kana by misrepresenting it as a challenge, anthropomorphizing and mourning a lost rock named "Yamada," and regaining belief in Santa Claus after receiving the bear from Fujioka disguised as Santa. Emotional vulnerability surfaces during family crises, like mistakenly believing Haruka would study abroad, leading her and Kana to unsuccessfully attempt household management before tearfully begging Haruka not to leave. She also experiences genuine grief over Fuyuki's departure and lingering attachment upon receiving his postcard.