Rei Hasekura stands out at Lillian Girls' Academy, first holding the position of Rosa Foetida en bouton before ascending to Rosa Foetida. Her tall, boyish appearance frequently causes others to mistake her for a boy, leading to the school nickname "Mr. Lillian." This bishōnen exterior contrasts with traditionally feminine interests and traits. She excels in domestic pursuits like cooking, baking, and knitting, creating the pink scarf and mittens worn by her petite sœur and cousin, Yoshino Shimazu. Reading shōjo novels and sightseeing are favored pastimes, with "sincerity" as her cherished word.

Beneath her imposing stature lies a personality marked by profound emotional vulnerability. Rei openly admits a deep dependence on Yoshino, describing her true self as weak, timid, and lonely without her. This manifests as obsessive care: ensuring daily walks home together, carrying Yoshino's bag when she's ill, and experiencing intense anxiety during separations. Rooted in childhood, where Rei acted as Yoshino's self-styled knight, this protectiveness sometimes causes friction as Yoshino dislikes excessive dependency. Rei can also display occasional insensitivity outside her close circle, such as mistakenly calling her date Chisato Tanuma by Yoshino's name.

Her bond with Yoshino defines her character arc. Rei formally becomes Yoshino's grande sœur by bestowing the Rosa Foetida en bouton rosary at her sickbed. This bond faces a severe test when Yoshino temporarily returns the rosary before the statue of Mother Maria after Rei arranged to leave school early for Yoshino's medical appointment without consultation. The rejection triggers a near-delirious emotional collapse, leaving Rei wandering the campus "like a ghost." Reconciliation occurs during winter near the statue after Yoshino's recovery, with Rei reaffirming her rosary vow.

Rei struggles to reconcile her protective nature with her personal identity. Fearing an inability to treat Yoshino impartially during the kendo club's rigorous barefoot winter practices, she initially forbids Yoshino from joining. When Yoshino joins secretly, Rei withdraws from school, consumed by worry about potential injury. A confrontation in the rain at the training hall leads Rei to confess her fear of showing either undue favoritism or excessive harshness. This conflict resolves as Rei acknowledges her ongoing need for Yoshino while accepting Yoshino's desire for independence.

Within the Yamayurikai, Rei supports peers like Yumi Fukuzawa, subtly encouraging Yumi's bond with Sachiko Ogasawara during a waltz rehearsal. She respects confidences, refusing to disclose details of a novel about Sei Satō's past relationship to protect Sei's feelings. Rei displays unwavering loyalty to her own grande sœur, Eriko Torii, following Eriko's decisions without argument, such as rejecting a Valentine's Day event proposal, and vehemently defends Eriko's reputation against false rumors in the school newspaper.

Key events highlight Rei's devotion. She wins a kendo tournament despite distress over Yoshino's hospitalization, motivated by Yoshino's wish for her participation. During Valentine's celebrations, she prioritizes Yoshino's comfort, designing a specialized apology cake after her date with Chisato Tanuma and promising visits to Yoshino's home. Rei also celebrates New Year with Yoshino and their families in Hakone. Later, unexpected circumstances lead her to humorously reference having a "ten-year-old boyfriend."