Nanaka Kirisato is the main protagonist of the series. She begins as a seventeen-year-old senior high school student who prioritizes studying above all else, viewing it as the most important thing in the world. This single-minded focus leaves her with few friends and a cold, distant demeanor; she is often described as self-centered and cynical. Due to circumstances, she gave up on her childhood early in life, adopting a serious, driven attitude toward academic success and college admission.

Her personality undergoes a drastic shift after a heated argument with her childhood friend Nenji Nagihara. Following the fight, Nanaka falls down a flight of stairs and suffers a brain injury that causes her memories and mental state to regress to that of a six-year-old child. In this regressed state, she becomes very spirited, energetic, and a troublemaker. She is obsessed with a childrens television show called Magical Domiko and frequently uses nonsensical phrases like hayaya and hawawa. Her appearance reflects this change, with her hair sometimes styled in a side pigtail and her eyes appearing wider and more innocent.

Nanakas motivation as a seventeen-year-old is centered on academic achievement and securing a future at a good college, driven by a desire to succeed that caused her to suppress her childhood. After the regression, her primary motivation becomes navigating the world with a childs curiosity and innocence while her father Taizo Kirisato and Nenji must keep her condition secret. She believes that Nenji made a wish to become an adult alongside her, which colors her interactions with him.

In the story, Nanaka serves as the central figure around whom the comedic and dramatic events revolve. Her dual personalities create conflict and growth for those around her, particularly Nenji, who is forced to mature and take on a caretaker role. Her key relationships include Nenji Nagihara, her childhood friend who becomes her primary guardian; her elderly father Taizo; Yuriko Amemiya, an honor student who initially resents Nanaka but later becomes a confidante and helps keep her secret; Jinpachi Arashiyama, a delinquent who mistakenly develops feelings for her; and Satsuki Arashiyama, Jinpachis younger sister who becomes fiercely protective and calls Nanaka her big sister.

Throughout the series, Nanaka undergoes significant development. The regressed six-year-old Nanaka gradually relearns social bonds, experiences friendship and love, and begins to rediscover the joy and warmth she abandoned as a teenager. Meanwhile, moments of the original seventeen-year-old personality occasionally surface, leading to a gradual integration of her two selves. By the end, she learns to balance her academic ambitions with emotional connections, growing into a more well-rounded person.

Notable abilities include her exceptional academic skills, which remain intact even in her regressed state; she is able to complete first-grade math workbooks on her own shortly after the accident. She also demonstrates surprising musical talent when she volunteers to play piano accompaniment for a choral competition, practicing diligently until she performs successfully on stage. Her childlike innocence occasionally allows her to inadvertently dodge attacks, as seen when she avoids Satsukis strikes while not understanding the danger. She has no supernatural or magical powers.