Ren Yamai attends Itan Private High School, progressing from Class 1-1 in her first year to Class 3-4 by her third. Hailing from considerable wealth, she lives independently in her own apartment, funded by her parents, and frequently travels internationally to destinations like Hawaii and Venice. Her name, 山井恋, carries a wordplay: "Yamai" can reference illness (病), while "Ren" means love, collectively hinting at "yandere". She presents a polished and poised exterior, always fashionable. She typically wears the school uniform with a cardigan instead of the blazer and brands a black ribbon in her auburn hair. An initial facade of a popular "mean girl" using a singsong voice masks a deeply aggressive and vitriolic core. This true nature erupts concerning Shouko Komi, whom she obsessively admires and lusts after, calling her "Komi-sama" or "Lady Komi." Her yandere obsession drives extreme actions: relentless stalking, collecting strands of Komi's hair, assembling photo collages, and engineering scenarios to touch Komi or glimpse her underwear, like proposing a haunted house to grope her in darkness. Her fixation fuels antagonism towards anyone close to Komi, particularly Hitohito Tadano and Omoharu Nakanaka. She once kidnapped Tadano, confined him in her room, and threatened to bury him in the mountains or dump him at sea, viewing him as an obstacle to Komi. Komi's furious rejection after this incident devastated Yamai, prompting tearful apologies. Though she reconciled with Tadano through Najimi Osana's mediation, jealousy lingered. She often collaborated with Komi's fanatics to harass him. Her rivalry with Nakanaka morphed into a reluctant camaraderie, marked by frequent bickering yet cooperation, like playing keyboard in the school band Perorabiosso during the cultural festival. When Komi and Tadano began dating, Yamai confronted Tadano in rage but retreated upon witnessing their mutual protectiveness. She secluded herself for a week before reluctantly accepting their relationship, vowing to "watch over" Komi despite inner turmoil. Her actions shifted toward comedic extremes—such as wearing Komi's tights on her head—while maintaining loyalty as a friend. She supported classmates academically and socially, helping Nakanaka integrate into class activities and tutoring peers in music. Academically, she claims disinterest in studying yet paradoxically achieves above-average grades, tying with Nakanaka at 36th in her second year and 28th in her third. She excels in English and sociology, aided by her international travel. Her talents include winning a violin competition and playing piano, contributing to the school band. Other students recognize her as the "priestess" of Komi’s fan group, reflecting her influence despite a divisive reputation. Her enduring friendships include Onigashima Akako, known since elementary school, and Najimi, met in middle school. While her family's opulence shaped materialistic tendencies, her development shows gradual integration into class dynamics, balancing obsessive traits with genuine, if unconventional, camaraderie.