Rena Ryūgū, born Reina Ryūgū, spent her childhood in Hinamizawa before relocating to Ibaraki Prefecture because of her mother's career. Her family life fractured when her mother began an affair with "Uncle Akihito," culminating in her parents' divorce. Rena vehemently opposed the divorce, especially after discovering her mother was pregnant with Akihito's child, fostering deep resentment towards them. She discarded all possessions connected to her mother, including family photos and furniture.

This family collapse triggered severe mental instability. In 1982, Rena suffered hallucinations of maggots beneath her skin, leading to self-harm. She later attacked three male classmates with a baseball bat during a school incident, permanently blinding one in one eye, and shattered numerous windows. This occurred after the students planned a haunted house based on real stories and sought her input, which she objected to. She faced no legal charges but underwent psychiatric evaluation, attributing her breakdown to a curse from Oyashiro-sama, the village deity, for abandoning Hinamizawa.

Rena and her father returned to Hinamizawa in April, where her mental state stabilized. She changed her name to Rena, removing the "i" from Reina to symbolize discarding the "icky" parts of her past. There, she joined the after-school club, befriending Keiichi Maebara, Mion Sonozaki, Rika Furude, and Satoko Hojo. Rena developed a habit of collecting items she considered "kyute" (cute) from the local junkyard, often exclaiming "Omochikaeri" ("I'll take it home!"). She frequently used phrases like "hau" and repeated words ending in "kana? kana?" ("I wonder?").

Her personality blended outward cheerfulness with intense perceptiveness. She demonstrated keen intuition, deducing events at Satoko and Rika's home with minimal clues. However, her belief in Oyashiro-sama's curse and aversion to lies could provoke violent episodes. When confronted about hidden information or if the curse's existence was questioned, her demeanor shifted dramatically, often marked by a loss of light in her eyes and aggressive behavior, sometimes wielding a hatchet or cleaver.

The console-exclusive arc "Tokihogushi-hen" (part of "Higurashi no Naku Koro ni Kizuna") explored Rena's Ibaraki background further. It featured her interactions with police investigator Tomoe Minai and former classmate Nagisa Ozaki, related to one of the boys Rena attacked. This storyline involved murders, including Nagisa and her boyfriend, linking back to Rena's past actions. Her trauma and the assault's aftermath resurfaced during the investigation.

In "Tsumihoroboshi-hen," Rena's perspective was central. Living with her father, she grew suspicious of his girlfriend, Rina Mamiya, believing Rina intended financial exploitation. After confirming Rina's deceit, Rena confronted her at a junkyard, resulting in Rena killing Rina and her accomplice Teppei Hojo. She methodically disposed of the bodies and nearly avoided detection, showcasing cold calculation under stress. This arc revealed her "kyute mode" as a deliberate tactic to deflect suspicion.

Rena maintained a close, competitive friendship with Keiichi, with underlying romantic tension. She occasionally received gifts from him, like a festival-won teddy bear. Her relationship with Mion was generally amicable, though Mion sometimes restrained Rena during obsessive episodes about Oyashiro-sama. Rena often targeted Rika and Satoko for their "kyute" qualities, especially when they wore punishment outfits from club games.

In later timelines, like "Higurashi no Naku Koro ni Gou," Rena attended Gogura University with Keiichi and Mion. By 2019, as depicted in "Higurashi no Naku Koro ni Rei," she had a son named Kihiro Ryūgū. Her attachment to collected junk persisted, viewing discarded items as "abandoned" and needing a home, reflecting her experiences of instability and displacement.