Ren Yamai attends Itan Private High School, introduced as a classmate of Shouko Komi and Hitohito Tadano in Class 1-1. Her surname, meaning "mountain" and "well," can also use the character for "illness," hinting at her yandere traits, while her given name "Ren" is an alternate reading of "koi," meaning "love." Born into significant wealth on November 8 (Scorpio), her affluent family supports her solo apartment living and frequent international travel to places like Hawaii and Venice. She prioritized beauty and popularity from a young age, forming a close childhood friendship with Akako Onigashima that continued into high school and befriending Osana Najimi in middle school, often helping them with homework. Physically, Yamai weighs 44 kg in her second year. She has long auburn hair with flyaways, typically styled with bangs and a black ribbon over her left ear. She favors the school uniform with a light brown cardigan instead of the blazer, knee-high socks, and a short skirt often concealed by the cardigan, adopting a more girly fashion style outside school. Yamai's personality presents a sharp contrast. Outwardly sociable, fashionable, and popular, she maintains a friendly, outgoing demeanor with classmates unrelated to Komi. Inwardly, she harbors a deeply obsessive and volatile fixation on Shouko Komi, whom she reveres as "Komi-sama." This manifests as yandere behavior, combining extreme worship with disturbing possessiveness, jealousy, and violent tendencies towards rivals. She collects Komi’s photos, creates collages, and fantasizes about intimate encounters, including scenarios involving groping or drugging. Her aggression targets Tadano most intensely, viewing him as obstructing her connection to Komi, and she clashes with Omoharu Nakanaka over Komi’s attention. Despite this, she genuinely cares for Komi’s well-being, rushing to aid her after a fall and filing down hazardous surfaces. In her first year, Yamai led a girls' group including Onigashima and Himeko Kishi and joined Komi’s circle. Her obsession prompted extreme acts, including kidnapping Tadano with intent to bury or drown him, leading to furious rejection by Komi. Following tearful apologies and Najimi’s mediation, she reconciled with Tadano and Komi, though her jealousy persisted. She continued scheming to isolate or physically interact with Komi, like proposing a haunted house to grope her. During her second year, she expanded her social circle, befriending Rumiko Manbagi’s gyaru group and assisting the band Pierro Rabioso as their keyboardist at the cultural festival. Academically, she ranked 36th in summer term exams despite claiming disinterest. By her third year, her grades improved to 28th, tying with Nakanaka. Yamai possesses notable talents, including proficiency in violin (winning a contest) and piano (playing keyboard for Pierro Rabioso). International travel bolsters her English skills, and she often incorporates foreign phrases into speech. She is resourceful in manipulating social situations, typically driven by her Komi obsessions. Her relationships are complex. She maintains childhood bonds with Onigashima and Najimi and leads a clique including Kishi. She oscillates between antagonism and reluctant cooperation with Tadano, acknowledging his communication skills with Komi while resenting their relationship. Her rivalry with Nakanaka evolves into contentious mutual respect, with Yamai using the nickname "Shishun" for her by their third year. Within the Komi fanatics, she is designated a "priestess." Yamai’s development shows incremental moderation. After learning of Komi and Tadano’s romantic relationship, she initially confronted Tadano but retreated upon witnessing their mutual protectiveness. She isolated herself for a week before resolving to "not accept, but also accept" their relationship. While obsessive impulses persist, she channels energy into supporting classmates like Nakanaka and sustaining band activities. Her actions increasingly serve comedic relief, and she demonstrates loyalty in non-Komi contexts, such as assisting peers with school adjustments. Despite lingering jealousy, she strives to manage her emotions, reflecting nuanced, though unstable, growth in her interactions.