Rei Hasekura is a notable second-year student at Lillian Girls' Academy, holding the position of Rosa Foetida en bouton before advancing to Rosa Foetida. Her tall, boyish build and bishōnen features earn her the nickname "Mr. Lillian" and cause frequent misidentification as male in casual clothes. Despite this appearance, Rei embraces traditionally feminine pursuits: baking, cooking, knitting, and reading shōjo novels. Her favorite word is "sincerity," and she enjoys sightseeing.

Cousins since childhood, Rei and Yoshino Shimazu share a profound bond, formalized when Rei became Yoshino's grande sœur at the academy. Rei exhibits intensely protective, caring, and obsessive devotion toward Yoshino. She hand-knits Yoshino's scarf and mittens, accompanies her daily walks to school, and carries her bag when she is unwell. Rei confesses that without Yoshino, her true self is "weak, timid, and a lonely crybaby." This dependency causes significant emotional turmoil; a temporary break in their sœur relationship leaves Rei wandering the campus "like a ghost," confiding her distress in strangers.

A kendo practitioner skilled enough to win tournaments, Rei's participation remains deeply linked to Yoshino. She initially opposes Yoshino joining the kendo club, fearing her own inability to avoid favoritism or excessive harshness. When Yoshino joins regardless, Rei avoids school temporarily. They later reconcile, with Rei acknowledging her partiality but affirming her undiminished feelings. Rei's protectiveness extends to shielding Yoshino from physical strain, discouraging rigorous kendo training due to concerns about morning practices and barefoot winter drills.

Rei defers to her own grande sœur, Eriko Torii, following her decisions without argument, recognizing that opposing Rosa Foetida is futile. She interacts respectfully with others, like Sei Satō, refusing to disclose details of Sei's past or a novel involving a romantic relationship between girls to protect Sei's privacy.

Subtle development occurs across Rei's story. Initially prioritizing Yoshino above all else, she blurts Yoshino's name during a date with Chisato Tanuma. She learns to navigate her protectiveness more healthily, ultimately supporting Yoshino's kendo participation despite anxieties. Rei demonstrates loyalty beyond Yoshino, angrily defending Eriko's reputation against false rumors. Later, she humorously references an "unexpected event" resulting in a ten-year-old boyfriend.

Rei's character balances strength and vulnerability—her kendo prowess contrasts with tearful moments during separation from Yoshino—highlighting themes of devotion and identity within the sœur system. Her growth centers on reconciling outward resilience with internal sensitivity while maintaining unwavering dedication.