Yumi Fukuzawa, a first-year student at Lillian Girls’ Academy with brown pigtails, bangs, and brown eyes, navigates a journey from bashful timidity to gradual self-assurance. Her bond with Sachiko Ogasawara begins when a photograph captures Sachiko adjusting Yumi’s uniform neckerchief, sparking their evolving dynamic. Though initially rejecting Sachiko’s offer to become her petite sœur due to perceived insincerity, Yumi accepts the rosary after witnessing Sachiko’s vulnerability over her arranged marriage to cousin Suguru Kashiwagi.

Within the Yamayuri Council, Yumi’s responsibilities grow, particularly in the fourth season as she faces pressure to choose her own petite sœur. Her interactions with Tōko Matsudaira, Sachiko’s cousin, shift from jealousy to nuanced collaboration as Yumi navigates Tōko’s emotional barriers and leadership potential. Strategic initiatives, such as organizing inter-school collaborations with the Hanadera Council, showcase Yumi’s ability to balance Sachiko’s autonomy with her aversion to male interactions.

Yumi’s empathetic influence extends to relationships like her exchanges with Sei Satō, whose playful requests for a farewell kiss challenge her boundaries. Sei’s acknowledgment of Yumi’s role in reigniting her academic drive underscores Yumi’s quiet impact. Recurring internal conflicts—fear of disappointing Sachiko, balancing loyalty with independence—surface during events like Valentine’s Day, where avoidance gives way to reconciliation through heartfelt gestures.

Her surname, meaning “fortune” and “marsh,” and given name, combining “help, assist” and “snake,” layer cultural nuance, while her March 29th birthday and trivia noting her as Sachiko’s second choice after Shimako Tōdō add depth. By the narrative’s conclusion, Yumi embodies a poised leader, her once-dependent demeanor refined into quiet authority, maintaining emotional connections while guiding others with collected grace.