Dear Brother (1991)

Misonoo Nanako, a professor's daughter, looks forward to her first day at the prestigious Seiran Academy, a girls' school that caters to the rich, or in her case, those with high test scores. She soon finds that the school is polarized by three popular and talented individuals, known as the Magnificent Three. Though Nanako doesn't intend to make waves at this school, when she is tabbed to be a member of the Sorority, the brutal politics that run beneath the surface of the school's sophisticated veneer soon turn Nanako's world into one where she is unsure of her friends, her enemies, and her very identity... The only way she can vent her emotions is through her correspondence letters to a man she calls her "Dear Brother".