Rider's true identity is Medusa of Greek myth, originally an Earth Goddess transformed into an anti-heroic monster by humanity's dark thoughts. She and her sisters Stheno and Euryale manifested from mankind's desire for ideal idols, dwelling on the Shapeless Isle. Unlike her immortal sisters, Medusa aged and grew tall, developing potent Mystic Eyes to guard them. Jealous Athena cursed Medusa alone, turning human adoration into hatred and forcing her exile. Protecting her sisters from invaders gradually corrupted Medusa, who began drinking blood instead of petrifying victims. Her sisters ultimately sacrificed themselves to her monstrous Gorgon form when she ceased to recognize them. The hero Perseus defeated her by reflecting her petrifying gaze with a mirrored shield and severing her head with the Harpe sword. Summoned as a Servant in the Fifth Holy Grail War by Sakura Matou, Rider serves under Sakura's unwilling proxy, her brother Shinji Matou. Rider feels no loyalty to Shinji but covertly protects Sakura. Her appearance diverges from the snake-haired Gorgon, featuring long purple hair and a blindfold-like "Mystic Eye Killer" visor sealing her petrifying gaze. Taciturn and analytical, Rider shares a profound bond with Sakura as fellow victims of monstrous transformation. She employs stealth and strategy in combat, wielding chain-linked iron nails and manipulating bounded fields. Rider commands three Noble Phantasms: "Blood Fort Andromeda," a bounded field dissolving humans to absorb life energy; "Breaker Gorgon," sealing targets within a mental prison; and "Bellerophon," summoning the Pegasus for a devastating light-based charge. Her combat style prioritizes mobility and tactical terrain use. During the Heaven's Feel route, Rider fully commits to Sakura after Shinji's demise. She allies with Shirou Emiya to save Sakura from the Shadow and Angra Mainyu's corruption. In the climactic battle, Rider combines Bellerophon with Shirou's projected Rho Aias shield, spearheading a combined assault that defeats the corrupted Saber Alter. Following Sakura's purification, Rider survives the Holy Grail War's conclusion. In the aftermath, she attains a permanent human form and continues her life alongside Sakura and Shirou, as seen in the cherry blossom viewing epilogue.