Ruri Ichigyō is a reserved first-year student at Kyoto Municipal Horikawa Senior High School, often perceived as unapproachable due to her silent demeanor and grumpy expressions. She habitually prioritizes reading over social interactions, using intimidating gazes to deter interruptions, reinforcing her loner status. Physically, she has dark blue-black hair in small side ponytails, yellowish-brown eyes, pale skin, and a distinctive mole on her left cheek. She wears standardized uniforms: a black sailor outfit with a pale brown ribbon in spring, a short-sleeved white version in summer, both with a black skirt, calf-length socks, and black leather shoes.

Her late grandfather's attic book collection deeply influences her philosophy that books "desire to be read" over preservation, leading her to donate them for a school event. She favors adventure novels for their themes of perseverance and determination—qualities she aspires to embody amid self-described loneliness. Her acrophobia surfaces during library duties.

Character development unfolds through interactions with classmate Naomi Katagaki, whom she meets via the Student Library Committee. Initially cold, she shares her home address instead of digital contacts. After slapping Naomi during a bus incident involving accidental contact, she apologizes when he returns her lost bookmark, beginning cautious trust. Their bond strengthens during shared tasks; when shelving books triggers her acrophobia-induced fainting, Naomi's assistance fosters mutual reliance. She later requests his help with technology (WiZ app), initiating digital contact exchange.

Preparations for the Secondhand Book Fair become pivotal. Inviting Naomi home to retrieve her grandfather's books, she discusses her love for adventure stories versus his sci-fi preference. A riverside conversation about sci-fi evoking "being part of the tale" prompts her first genuine smile. After a fire destroys the donated books—including her cherished copy of "The City at the Bottom of the Great Lake"—Naomi's restoration efforts inspire her confession: "Dating is something you can’t do alone. So... let’s do it together, shall we?"

Her background incorporates a sci-fi twist: within the quantum storage system ALLTALE, she exists as recorded data. The "real" Ruri suffered a lightning-induced coma during a fireworks festival. Future Naomi manipulates this simulation to synchronize her consciousness with her physical body. When ALLTALE's homeostasis attempts to erase her unstable data, both Naomis collaborate to transport her to safety. She awakens in a reconstructed world with teenage Naomi, implying a stable future.

The spin-off "Another World" contextualizes her as the emotional catalyst for Future Naomi's actions across timelines. Though minimally depicted, it reinforces her significance. Her development concludes with increased social openness, including a friendship formed with classmate Misuzu Kadenokōji during book fair preparations.