Rei Hasekura stands as a prominent student at Lillian Girls' Academy, holding the position of Rosa Foetida en bouton within its Yamayuri Council student government. Her notably boyish, tall appearance and bishōnen looks earned her the title "Mr. Lillian." She sports short blonde hair and brown eyes, frequently donning attire that causes others to mistake her for male. This outward tomboyishness sharply contrasts her private enjoyment of traditionally feminine pursuits like baking, cooking, knitting, and reading shojo novels. Her favorite word is "sincerity," and she relishes sightseeing. Beneath her masculine presentation, Rei reveals a gentle, emotionally vulnerable core, admitting she becomes "weak, timid, and lonely" without her petite sœur and cousin, Yoshino Shimazu.

Rei and Yoshino, cousins and lifelong next-door neighbors, share a bond predating their formal sœur pairing at Lillian. As Yoshino’s grande sœur, Rei bestowed her with a rosary to solidify their relationship. Rei exhibits obsessive care and overprotectiveness toward Yoshino, earning her the label "worrywart." She crafts scarves and mittens for Yoshino, constantly prioritizes her welfare—even during kendo tournaments—and once accidentally called Yoshino’s name during a date with Tanuma Chisato. This dynamic fuels friction, as Yoshino cherishes independence and resents Rei’s smothering attention. Rei grapples to reconcile her nurturing instincts with Yoshino’s autonomy, exemplified when she initially barred Yoshino from the kendo club over fears of distraction or favoritism. Rei acknowledges being "powerless" against Yoshino’s strong will, often finding her "orders" treated as mere requests.

A skilled kendo practitioner who competes in tournaments, Rei’s athletic involvement bolsters her tough exterior while underscoring her emotional reliance on Yoshino. Her overprotectiveness intensified during Yoshino’s recovery from heart surgery, though Yoshino’s improved health later allowed their relationship to evolve. Yoshino’s growing independence forces Rei to confront her vulnerabilities, particularly her dread of loneliness. Rei’s own grande sœur is Eriko Torii; their scant documented interactions show Rei following Eriko’s directives without dispute, conceding that arguing with Rosa Foetida is futile.

Rei undergoes gradual character growth throughout the series. Her early dependence on Yoshino shifts toward mutual interdependence, highlighted during an Italy trip where Yoshino confides to Yumi Fukuzawa that Rei is among the few she trusts with her weaknesses. Rei also displays loyalty to peers like Sachiko Ogasawara, offering emotional support during Sachiko’s personal struggles. Later seasons note Rei acquiring a "ten-year-old boyfriend" via "unexpected events"—a minor but clear sign of her social world expanding beyond Yoshino.

Public perception often skews Rei’s image; the Newspaper Club mistakes Yoshino’s boyish hobbies (such as reading Ikenami Shoutaro’s books) for Rei’s and vice versa, resulting in erroneous publications about the Rosa Foetida lineage. This mix-up emphasizes the gap between Rei’s masculine appearance and her authentic, softer personality.