Sonoe Jūjō is a popular and stylish student distinguished by her tanned skin, light blonde hair, and green eyes, often highlighted with makeup. She leads a group of similarly flashy classmates, amplifying her influential presence at school. Initially, her teasing and boastful demeanor—especially about romantic experiences—creates friction with classmate Rika Sonezaki, who resents Sonoe’s open attitude toward sexuality and perceives her as a "mean girl." This dynamic shifts as Sonoe reveals self-reflection and regret when her teasing goes too far, exposing a fundamentally kind, carefree personality beneath her confident exterior. Her relationship with Rika evolves from antagonism to genuine friendship, marked by Sonoe affectionately nicknaming her "Osone" and becoming her only true friend.

Sonoe maintains a relationship with a gentle boyfriend whose unexpectedly soft, androgynous appearance surprises others. Their bond becomes central to a major turning point: during the school’s cultural festival, they are seen together, and Sonoe soon discovers she is pregnant. This leads to her pre-graduation departure. Before leaving, she gives Rika a handmade star-shaped cookie and a note expressing gratitude and farewell, symbolizing their friendship. Her exit prompts the school administration to implement an abrupt rule banning relationships between male and female students, causing significant disruption and conflict among students.

After departing, Sonoe does not reappear at school. The narrative later reveals she has given birth and lives happily, caring for her child alongside her boyfriend. This positive resolution underscores her character arc. Her pregnancy and exit act as catalysts for key events and thematic explorations, profoundly impacting other characters and the school environment.