Rinko Koujiro, a scientist and former colleague of Akihiko Kayaba, collaborated with him on developing Sword Art Online during their studies at Toto Institute of Technology. Meeting Kayaba in her first college year, she joined the SAO project unaware of his deadly intentions for the game. When players became trapped, she deduced Kayaba's location and traveled to his mountain cottage to confront him, but instead maintained his physical condition during his extended dive. After Kayaba's death, she faced arrest but secured acquittal partly due to a fake micro bomb implanted near her sternum—Kayaba's ploy to suggest coercion.

Following the SAO incident, Rinko moved to San Francisco. Seijirou Kikuoka contacted her for Rath’s confidential FullDive research, but she declined. On July 1, 2026, after receiving an email from Asuna Yuuki detailing Kirito’s critical condition and disappearance, Rinko agreed to assist. She contacted Kikuoka and traveled with Asuna—disguised as her assistant "Mayumi Reynolds"—to Rath’s Ocean Turtle facility. There, she learned of the Soul Translator (STL) and Underworld project, which created artificial fluctlights for advanced AI. Her background in developing the Medicuboid, derived from Kayaba’s work, enabled her to engage with Rath’s initiatives.

During the Ocean Turtle’s invasion in the War of Underworld, Rinko remained aboard. She provided technical insights on the STL and Underworld’s accelerated time flow—where one real-world day spanned simulation years—supporting crisis stabilization and protection of artificial fluctlights, including Kirito’s damaged consciousness. Though not combat-involved, her presence aided Rath’s operational continuity throughout the conflict.