Ichigyou Ruri is a high school student at Kyoto Municipal Horikawa Senior High School. She appears silent and reserved, preferring to read books and often using reading to avoid social interaction. This behavior leads classmates to perceive her as grumpy and unapproachable, leaving her a loner. She has pale white skin, yellowish-brown eyes, and dark black-blue hair usually tied in small ponytails on both sides with the rest loose. A mole sits on her left cheek. Her standard school attire includes a black sailor uniform with a pale brown ribbon and black skirt for spring, and a white short-sleeve sailor uniform with a black skirt for summer, worn with calf-length socks and black leather shoes. At home, she wears a white sleeveless shirt, blue short pants, and a yellow cotton jacket.

Ruri's character shifts when she interacts with classmate Katagaki Naomi through shared Student Library Committee duties. Though initially reluctant, she gradually opens up after he returns a dropped bookmark, marking the start of their relationship. Naomi's consistent kindness helps her overcome isolation. She begins trusting him, exchanging contact information and collaborating on library tasks like organizing books and preparing for the Secondhand Fair. During this time, she reveals her fear of heights and fondness for adventure novels, admiring their themes of perseverance. Her emotional barriers lower further as she shares moments of vulnerability, fainting during library duty and accepting Naomi's assistance.

A significant aspect reveals Ruri exists as recorded data within the quantum computer system ALLTALE. In the original timeline, lightning struck her during a fireworks festival after beginning a relationship with Naomi, resulting in a coma. This event prompts the adult Naomi from 2037 to intervene in the simulated 2027 Kyoto to alter her fate. The adult Naomi ("Sensei") forcibly transfers Ruri's data out of ALLTALE, attempting to synchronize it with her comatose real-world self to revive her. This action triggers the ALLTALE's Homeostatic System, seeking to erase her as anomalous data. During the conflict, the younger Naomi rescues her, and they return to their simulated world together. An additional layer of reality shows an older Ruri living in a lunar habitat interacting with an older Naomi awakening from a coma, implying further complexity across recursive realities.

Her relationship with Naomi culminates in a mutual romantic confession where she states, "Dating is something you can’t do alone. So... let’s do it together, shall we?" This signifies her full emotional transformation from an isolated individual to one capable of intimate connection, underlining her journey from a detached bookworm to someone who values companionship, driven by Naomi's unwavering support and understanding.