Rena Ryūgū, originally named Reina Ryūgū, endured profound family turmoil during childhood. Her parents divorced due to her mother's affair, triggering a mental breakdown. She attacked three male classmates with a baseball bat in Ibaraki, inflicting permanent blindness on one and shattering school windows. Hospitalized after the incident, she asserted Oyashiro-sama commanded her return to Hinamizawa for safety. This event resulted from terminal symptoms of Hinamizawa Syndrome, manifesting as delusions of maggots infesting her blood and prompting self-harm. Subsequently, she altered her name to Rena, omitting the "i" from Reina to symbolically purge "icky things" from her existence, though she later recognized that "incredible things" also begin with "i". Her father relocated with her to Hinamizawa in April, where her mental condition stabilized.

In Hinamizawa, Rena exhibited a cheerful, friendly demeanor coupled with an intense fixation on cute objects or people. She frequently entered "Cute Mode" or "Take Home Mode," declaring her intent to bring items home while displaying heightened physical agility. She pursued "treasure hunting" at the village junkyard, curating a secret base inside an old van filled with collected finds. Despite her typically kind nature, she reacted violently to lies or challenges to her belief in Oyashiro-sama's curse, epitomized by her iconic "Uso da!" outburst during confrontations over deception. Rena demonstrated sharp perceptiveness, accurately deducing situations from minor clues and discerning others' sincerity through eye contact. She habitually carried a cleaver or hatchet, usually sourced from the junkyard, employing it as a weapon in extreme circumstances.

Her backstory involved deep trauma. Rena blamed herself for her parents' divorce and believed Oyashiro-sama punished her for leaving Hinamizawa. In Tsumihoroboshi-hen, she killed scam artists Rina Mamiya and Teppei Hōjō to shield her father from their plots, methodically disposing of their bodies. This culminated in a school hostage crisis where she threatened to detonate explosives, fueled by paranoia after reading Takano Miyo's scrapbook. Keiichi Maebara intervened, averting further disaster. Rena maintained complex relationships, notably a competitive yet potentially romantic dynamic with Keiichi and a protective friendship with Satoko Hojo. Loyalty to friends and family frequently motivated her actions, though Hinamizawa Syndrome could intensify her violent tendencies.

In the OVA series Rei, Rena appeared in distinct contexts. The fifth episode, "Hirukowashi-hen," depicted a supernatural magatama incident. A broken shrine seal released a red magatama that fell into Rena's mouth, compelling her to fall in love with the holder of the matching white magatama. As the white bead circulated among characters, Rena's affections shifted abruptly, triggering chaotic romantic pursuits. Resolution occurred only after returning both beads to the shrine for a ceremonial ritual, highlighting her persistent "Cute Mode" tendencies. Additionally, Rei featured future segments set in 2019, revealing an adult Rena as the mother of Kihiro, a central character. In other sequels like Higurashi no Naku Koro ni Gou, Rena appeared as a university student years after the main events.

Rena's character arc concluded with her achieving normalcy. She overcame the cyclical tragedies driven by external forces and her own psychological struggles. Her journey culminated in societal integration as a well-adjusted adult, moving beyond her traumatic past toward a peaceful future.