Rider is the Rider-class Servant of Sakura Matou in the Fifth Holy Grail War of Fate/stay night. Due to Sakura's unwillingness to participate as a Master, Shinji Matou is instead given control over Rider. She only truly acts as Sakura's Servant when their connection is revealed in the Heaven's Feel scenario. She is one of the Servants of the Protagonist of the Grand Order conflicts of Fate/Grand Order.
Rider's True Name is Medusa, the Gorgon from Greek mythology. Rather than being a regular Heroic Spirit, she is an existence closer to a Divine Spirit, but her actions have made it better to categorize her as an "anti-hero closer to a monster." With her older sisters Stheno and Euryale, they made up the three Gorgon Sisters who inhabited the Shapeless Isle. She was known as the "woman who dominates", an anti-hero who became as such due to the dark thoughts of humans. Differing from someone like Hassan-i-Sabbah who was a hero who actually existed, she is said to be one of legends born by the gathering of belief.
The legend of Medusa is different than her actual life. She was said to be a half-human, half-god female beast often portrayed with the form of a snake. Originally she was an Earth Goddess.
While all originally beautiful women, she and her sisters were hated by Poseidon's wife because of his love for them. Athena, jealous of their beautiful hair, transformed them into demons. Secluding themselves in a lightless temple on the Shapeless Isle, they only attacked humans with a reason. Eventually, as a victim of the gods' unreasonable actions, she was to have her head cut off on their whim. Defeated by Perseus by reflecting her petrification back at her with his mirror-like shield, Pegasus, the son of Poseidon, was said to have sprung from the blood of her headless neck.
Medusa and her sisters were born from the wishes of mankind for ideal idols, the embodiment of the yearning hearts of men. As "perfect goddesses", their "divine essence" was to remain the same from the moment of their creation to their destruction. Medusa was an exception who was a failure from birth, growing older over time. Though they should have all been copies, or even as close as clones, resulting from the deification of the same phenomenon, a mistake in her birth or something of deeper significance caused her to be copied imperfectly. While she did not embody "forever young and immortal", she gained abilities in compensation compared to her sisters' inability to even care for themselves. With divine powers, her Mystic Eyes, indispensable in protecting her sisters who could not live in the world on their own, this caused the previously united sisters to become three separate individuals. Although Medusa was different from the start, even the other two showed irregularities, their sadistic nature, from interacting with her.
She was originally considered to be a princess loved by humans, but Athena, jealous of their beauty, cursed Medusa alone, taking away her "faith from each and every person" and exiling her to the Shapeless Isle. Their love for her turned to hatred, and she was banished by those who had adored her. The island had nothing to offer compared to her days of luxury, and she prayed that none of the humans that now frightened her with their unwarranted resentment would tread on the island. Weeping in loneliness, she had no idea as to her sin that forced her to be exiled. Although her sisters were not affected and still greatly loved, they too accompanied her there out of caring for their younger sister. She was happy even on the barren island due to their company, and only had to fear the coming humans.
While they became rumored as monsters, her sisters were still revered by nearby men, while she was feared as a monster. Humans sought to kill that which had been branded as a monster and take away the goddesses, and after they began to also target her sisters, she decided to protect them while taking her revenge on the humans. Although littered with statues that had been warriors, they were harmonious for a time on the island, at one point receiving her Pegasus from Poseidon. Though bullied by her sisters daily, she simply moved about the island on days humans did not appear, hoping for the peace to continue. Repaying them in the same manner as they treated her, she killed many humans, simply hoping to be left alone on the only place she could exist with the company of her sisters.
Though she was protecting them from the foolish humans who sought their own deaths, her sisters warned her not to take pleasure in killing them. Using her eyes that were only a deterrent to sow terror in humans, she began to experiment in ways of killing them more quickly. Deciding to become a trap against those who would take her sisters, even they who had always been full of joy began to fear her after she started drinking their blood instead of simply petrifying them. The humans continued to journey the island no matter the infamy surrounding her, and she eventually became too ashamed of her slowly disfiguring self to appear before her sisters. Leaving them, she once again found herself alone, hungering for new sacrifices to replace her sadness and loneliness.
She grew into the Gorgon each day as her body, heart, and very being decayed, all backed by the thought of becoming stronger for the sake of protecting her sisters. Even that reached a point where her sisters instead became nuisances who were intruding upon her lair. They appeared before her after her full descent into a monster, lamenting that they had not asked for her protection and were just letting her enjoy what had seemed fun to her. Saddened by her state, they sacrificed themselves to the Gorgon because the sister who protected them was no longer there. Consumed by countless snakes without any trace of joy or sadness, they were ground into pulp and absorbed into the beast while still alive. Without leaving a trace, they became its flesh and blood, and it continued to thrive as the Gorgon sisters became monsters of nightmares and legends.
The Gorgon's notoriety continued until Perseus decided to bring back its head for his own fame. Granted five Noble Phantasms, he boldly traveled to the Shapeless Isle without any hesitation, only realizing his folly upon seeing the state of the island. He despaired upon encountering it, having decided that there is no victory to be found against the demon god, with his Noble Phantasms only acting as lifesaving equipment in the best scenario. Avoiding her gaze by focusing on his Mirror shield, he desperately fought to survive. Tired of toying with him, it used Breaker Gorgon, and he used the only possible method of defeating it, using Kibisis to have it encage itself.
Reversing the nightmare of the Bounded Field upon it, it suffered as Medusa as the nightmare reflected upon it instead of Perseus. While it was a "nightmare beyond nightmares", it was not an evil vision that caused excruciating pain, but a pleasant dream of her sisters, paralyzing her with the joy of seeing those she thought she would never again meet. Despairing upon the reflection of itself from when it was still self-aware, this allowed Perseus to have his single counterattack, severing its head with Harpe. The monster, out of its own will, became Medusa again and let itself be struck down. Perseus used Kibisis to hold its head, later going on to become a successful hero.
And she is reprised by Karen Strassman in the English dub and Yu Asakawa in the Japanese dub.