Rei Hasekura, a second-year student at Lillian Girls’ Academy, begins as Rosa Foetida en bouton under her grande sœur Eriko Torii before ascending to Rosa Foetida in her third year. Her boyish charm and bishōnen features earn her the moniker “Mr. Lillian,” though her demeanor contrasts sharply with traditionally feminine pursuits like knitting, baking, and immersing herself in shōjo novels. Guided by her favorite word, “sincerity,” she approaches relationships and duties with earnest devotion.

Rei shares an unbreakable bond with her cousin Yoshino Shimazu, forged through years as self-proclaimed protector and later as soeurs. From childhood, Rei adopted the role of Yoshino’s knight, carrying her schoolbag and shielding her from harm—a dynamic that spirals into codependency. She crafts Yoshino’s winter accessories, prepares Valentine’s chocolates, and prioritizes her needs above all else. Their bond fractures when Yoshino returns her rosary during a hospitalization, rejecting Rei’s suffocating care. This rupture forces Rei to confront her insecurities: a fear of inadequacy and the paralyzing dread of life without Yoshino.

In kendo, Rei’s disciplined leadership as team captain clashes with personal turmoil when Yoshino joins the club. Struggling to balance impartiality and protectiveness, she confesses during a rain-drenched confrontation that Yoshino’s presence unravels her focus—yet vows unwavering loyalty. This raw admission signals her first step toward reconciling duty with vulnerability.

As a Yamayuri Council leader, Rei navigates Eriko’s legacy, initially complying with her mentor’s expectations rooted in Rei’s androgynous appeal. Gradually, she asserts independence, notably by quashing rumors about Eriko’s romantic life. Upon becoming Rosa Foetida, she mentors her petite sœur while upholding traditions like Valentine’s events and election oversight, blending reverence for the past with emerging self-assurance.

A fleeting half-day date with Chisato Tanuma during a school festival underscores Rei’s fixation on Yoshino when she accidentally calls out her cousin’s name. Their reconciliation culminates in a cake baked by Rei and a renewed rosary exchange at the Virgin Mary statue—a ceremony initially rushed due to Yoshino’s illness, now imbued with deeper emotional resonance.

Rei’s perceptiveness shines in interactions beyond Yoshino: she gently nurtures Yumi Fukuzawa’s admiration for Sachiko, drawing from her own relational struggles, and safeguards Sei Satō’s privacy by refusing to expose ties to a controversial novel, showcasing loyalty and discretion.

Her journey traces a shift from self-doubt-riddled caregiver to a leader embracing vulnerability. Critical milestones—the collapse and revival of her soeur bond, candid kendo anxieties, and prioritizing emotional authenticity over stoic façons—mark her evolution, framing a narrative of resilience forged through introspection and connection.