Medusa, known as Rider in the Fate/stay night series, is a Servant summoned during the Fifth Holy Grail War. She is renowned for her beauty and tragic history from Greek mythology, where she was originally a beautiful maiden transformed into a Gorgon. In the series, Medusa serves as the Servant of Sakura Matou, and her abilities include using 'Mystic Eyes' to petrify opponents. Despite her fearsome reputation, she is a loyal and protective Servant, deeply caring for her Master, and plays a crucial protective role throughout the storyline.