Rider serves as a Sakura Matou's Servant in the Fifth Holy Grail War, though command transfers to Shinji Matou due to Sakura's reluctance. Her true identity is Medusa, one of Greek mythology's three Gorgon sisters. Born from mankind's wishes for ideal goddesses, Medusa differs from her immortal sisters Stheno and Euryale by being mortal and aging. Originally an Earth Goddess, she suffers a curse from Athena that strips away human devotion, leading to her exile on the Shapeless Isle. Isolated and feared as a monster, she transforms into the Gorgon while attempting to protect her sisters, ultimately consuming them unintentionally. Perseus defeats her by exploiting her momentary paralysis upon seeing her sisters' reflection. In human form, Rider possesses long purple hair and stands 172 cm tall. She typically wears a blindfold-like visor called "Breaker Gorgon" to seal her Mystic Eyes of Cybele, which induce petrification. Her primary weapon is a chain bearing metallic spikes resembling large nails. Her personality manifests as silence, vigilance, and logical analysis. Though outwardly impassive, she exhibits strong protectiveness, particularly toward Sakura, and harbors deep disdain for Shinji, whom she considers cowardly and predatory. Despite Shinji's official Master status, Rider's actions consistently prioritize Sakura's well-being. During a confrontation in Heaven's Feel II. lost butterfly, Shinji attempts to leverage Sakura as a hostage. When Sakura expresses distress, Rider definitively shifts her loyalty, rejecting Shinji's commands and acknowledging Sakura as her true Master. She subsequently defends Sakura from Rin Tohsaka and Archer, utilizing her Mystic Eyes offensively for the first time in the narrative. This intervention halts only when Sakura self-harms due to Shirou Emiya's injury, demonstrating Rider's prioritization of Sakura's immediate safety over combat. Following Sakura's refuge at the Emiya household, Rider remains as her guardian. She gradually develops cautious trust in Shirou, recognizing his commitment to protecting Sakura. However, she maintains constant vigilance over Sakura's deteriorating mental and physical state, exacerbated by the Matou family's Crest Worms and the emergence of a corrupting shadow within her. Rider's protective instincts intensify as Sakura's stability wanes, culminating in Sakura instructing Rider to prioritize Shirou's protection above all else. Rider's capabilities include exceptional mobility, agility, and proficiency with bounded fields. Her Noble Phantasms encompass Blood Fort Andromeda, a bounded field converting trapped organisms into mana, and Bellerophon, empowering her summoned mount Pegasus for a devastating charge attack. Her combat style emphasizes tactical use of terrain and covert engagements, compensating for Shinji's inability to supply sufficient mana. Her Mystic Eyes remain a potent defensive and offensive tool when unsealed.