Hailing from the wealthy and prestigious Ogasawara family, Sachiko is often perceived as a "Japanese Princess," her upbringing isolating her from common experiences like shopping or fast-food dining. An arranged childhood marriage to her cousin Suguru Kashiwagi, stemming from her own kindergarten declaration, caused her distress due to Suguru's inability to return her feelings as he was gay. She eventually dissolved the engagement by confronting her parents about its unfairness, revealing they had mistakenly assumed mutual affection. Her personality blends inherited traits: stubbornness and humor from her father, and strict, ladylike manners from her mother. Witnessing her father and grandfather openly keep mistresses fostered a deep misandry, though she made exceptions for Yumi's brother Yuuki and her aversion to Suguru lessened over time. Her grande sœur, Yōko Mizuno, selected her as a petite sœur to free her from family constraints, leading Sachiko to abandon activities like ballet. Initially presenting as cold, aloof, and reserved—Lillian Joshigakuen's idol—Sachiko possesses underlying vulnerability, protectiveness (especially towards Yumi), and emotional complexity. She exhibits jealousy, notably around Sei Satō when Yumi interacts comfortably with her, and embarrassment manifesting as mild anger. Her relationship with petite sœur Yumi Fukuzawa gradually softens her into a more carefree personality, though she retains a tendency to conceal personal problems, causing misunderstandings. Key aversions include losing, crowded places, summer heat, and inauthentic or status-obsessed individuals. She also harbors specific phobias: acrophobia and anxiety around moving vehicles, though she temporarily overcame her fear of heights during a critical moment at Hanadera's school festival to reach Yumi. Sachiko's development centers on her bond with Yumi. After impulsively offering Yumi her rosary—partly motivated by avoiding a play role with a male—and facing initial rejection, Sachiko persevered to genuinely earn Yumi's acceptance. Yumi guided her through "common people" experiences like selecting jeans and fast-food dining, while Sachiko grew increasingly protective and emotionally reliant. Demonstrations of their deep connection include Sachiko descending a two-story ladder despite acrophobia to reach Yumi in a panda costume, and their piano-vocal duet of "Maria-sama no Kokoro" at a summer villa party, thwarting acquaintances' attempts to humiliate Yumi. As Rosa Chinensis in her third year, Sachiko encouraged Yumi to seek her own petite sœur. Temporary focus on her cousin Touko Matsudaira sparked Yumi's jealousy, and her grandmother's death triggered a depressive episode resolved only by Yumi's heartfelt reassurance. Sachiko also learned pragmatic navigation of her misandry, participating in meetings with Hanadera's all-male student council after Yumi's encouragement, acknowledging future unavoidable interactions with men. Significant relationships define her journey. Her bond with Yumi evolved from formal mentorship to deep mutual dependence, with Sachiko adopting phrases and habits from her grande sœur Yōko. While Rei Hasekura reflected Sachiko's profound feelings weren't romantic, their relationship featured strong pseudo-romantic undertones and intense loyalty. Her dynamic with Suguru Kashiwagi transitioned from initial affection and bitterness to amicable understanding after the engagement's dissolution and acknowledgment of his sexuality. Interactions with peers like Rei included supportive gestures, such as hugging Rei during conflicts about balancing personal aspirations with Yoshino. Sachiko's growth is marked by gradual emotional openness, resilience confronting familial and personal challenges, and the softening of her rigid exterior through meaningful connections.