Riri Hitotsuyanagi hails from Yamanashi, Japan, born into a family renowned for their crystal workshop. Her father, Hiroshi, served formerly in the JGSDF, while her cheerful mother, Rika, passed on her bright demeanor. Riri has a younger brother, Hikaru, residing in an evacuation zone after their displacement. From childhood, Riri showed exceptional athleticism, achieving top scores in prefectural fitness tests. Despite her father's hopes and suggestions she enroll at Kaiseizan Garden to become a Lily, she initially had no interest in combat and planned to attend a civilian high school.

The Huge attack during the Koshu Evacuation irrevocably changed her path. Rescued by Yuyu Shirai, Riri found her inspiration to become a Lily. She applied to Yurigaoka Girls' Academy but scored second lowest among applicants, barely securing admission through the waitlist as a "booby prize" recipient. Academics remained a struggle, requiring intensive remedial sessions with peers.

Personality-wise, Riri maintains a bright, cheerful, and optimistic outlook regardless of hardship. She cares deeply for others, often placing herself in danger to protect strangers or teammates. This protective recklessness manifests in combat; though she typically provides fire support from the BZ position, she frequently charges into close combat when others are threatened. Her athletic endurance sustains her effectiveness in prolonged battles. She holds a particular aversion to insects, especially cockroaches, and carries multiple Ramune bottles in her bag, enjoying junk food like hamburgers. Her hobbies include physical exercise and collecting crystals, reflecting her family heritage.

Riri wields the versatile Gungnir CHARM, capable of both suppressive fire and melee combat. Despite her late start as a Lily, she learns quickly and adapts well. Her Rare Skill, Charisma, purifies and stabilizes spirits while amplifying Magie to boost legion morale and performance. This skill evolved into Laplace, a higher-level ability connecting thoughts into power by linking Magie across individuals. Her combat style leverages innate reflexes; she instinctively reacts to threats, such as saving teammates during Huge encounters despite lacking formal experience.

Key relationships define her development. She sought out Yuyu Shirai at Yurigaoka, eventually becoming her Schutzengel. Yuyu's trauma from her previous Schutzengel's death initially created emotional distance, but Riri persistently supported her, using "Cooldown Hugs" during Lunatic Trancer episodes. Their bond solidified when Yuyu gifted her a Brionac CHARM and a treasured four-leaf clover accessory. Riri formed Team Hitotsuyanagi, uniting diverse Lilies including Tazusa Andō, whom she helped overcome distrust rooted in a dark past involving GEHENA experiments.

Riri's leadership emerged during critical missions. She spearheaded the search for Yuyu's lost hairpin, mobilizing the entire academy through Laplace to amplify efforts. In the battle against an evolved mega-Huge, her Charisma merged with residual energy from Yuyu's former partner, neutralizing a Magie-dampening field and enabling a united Neunwelt Tactic victory. She later volunteered for a high-risk mission to destroy an ultra-class Huge, descending with Yuyu to inject destructive code via the Dáinsleif CHARM. Following the Huge's defeat, Yuri's intervention safeguarded them in a dream sequence, washing them ashore to a communal celebration.

The mysterious artificial Lily, Yuri Hitotsuyanagi, profoundly impacted Riri's journey. Discovered on a beach, Yuri was named by Riri as an homage to Yuyu. Riri shielded her from government capture despite the risks. Yuri's self-sacrifice against a Huge left Riri grieving. During solitary confinement for defying orders, Riri processed this loss privately before accepting support from her legion and schoolmates.