Rei Hasekura attends Lillian Girls' Academy, initially serving as Rosa Foetida en bouton before succeeding Eriko Torii as Rosa Foetida in her second year. Her tomboyish appearance and bishounen features earn her the nickname "Mr. Lillian," contrasting sharply with her personality. Rei possesses traditionally feminine interests like reading shoujo novels, baking, cooking, knitting, and sightseeing, frequently using the word "sincerity." She handcrafted the mittens and scarf worn by her petite soeur, Yoshino Shimazu. Rei and Yoshino are blood-related cousins who grew up in neighboring households, deepening their soeur bond. Rei carries Yoshino's school bag daily and exhibits obsessive concern for her well-being, often prioritizing Yoshino's needs to the point of self-neglect. This overprotectiveness stems from Rei's admission that without Yoshino, her true self feels "weak, timid, and lonely." Her intense dependence creates tension, leading to tactless remarks that prompt Yoshino to temporarily return their bond's symbolic rosary. Their conflict culminates in a formal re-presentation of the rosary at the Virgin Mary statue, an emotional significance Rei initially struggles to grasp. As kendo team captain (2nd Dan), Rei trains younger members and competes regularly. Yoshino's decision to join the kendo team triggers Rei's anxiety about showing favoritism or failing to maintain her composed facade. This dynamic illustrates Rei's struggle to balance her identity: her kendo passion represents personal independence, while interactions with Yoshino reveal deep-seated vulnerabilities. Friendships, particularly with Sachiko Ogasawara, provide crucial support; Sachiko offers direct advice during Rei's conflicts with Yoshino and comforts her during uncertainty about the future. Rei's relationship with her grande soeur, Eriko Torii, is defined by unquestioning obedience. Eriko selected Rei as petite soeur out of curiosity about her boyish appearance, and Rei typically capitulates to Eriko's requests without resistance—a habit that frustrates Yoshino. Following Eriko's graduation, Rei assumes leadership of the Rosa Foetida family within the Yamayuri Council alongside Sachiko and Shimako Tōdō. Her popularity ensures a smooth transition into this role demanding greater emotional maturity. Throughout her development, Rei gradually learns to voice fears and insecurities, especially concerning her future with or without Yoshino. A pivotal open conversation with Yoshino allows Rei to confront self-imposed pressures, fostering gradual emotional independence while preserving their deep bond.