At the prestigious Seiran Academy, 16-year-old Nanako Misonoo envisions a promising future, only to plunge into a world of lies, sorrow, and envy. Handpicked to join the Sorority, an esteemed circle at the school, Nanako, lacking the elegance and affluence of her peers, becomes the target of intense jealousy, notably from the formidable Aya Misaki. Struggling to navigate her tumultuous school life, Nanako finds solace in reminiscing about her past with her beloved teacher, Takehiko Henmi, whom she fondly refers to as 'onii-sama' (big brother). Supported by her loyal friends—childhood companion Tomoko Arikura, vivacious Mariko Shinobu, troubled musician Rei Asaka, and sporty Kaoru Orihara—Nanako's journey unfolds through poignant letters and the trials of bullying, unveiling the harsh realities of her school days in Oniisama e....