|Masked | Unmasked | Student | hollow ataraxia|
|Appears in:|| Fate/Prototype|
|Voice actor:||Mamoru Miyano|
|Character type:||Servant (Master: Isemi)|
"For him, who wished for the happiness of everyone... ---- For his sake, I will kill you all."
Rider is Perseus (ペルセウス, Peruseusu?), the Greek hero known for killing the Gorgon Medusa with Harpe and later becoming the king of Mycenae. He is the son of Zeus, and the warrior most trusted by the gods at the time in the Greek world. He sought to challenge the monster to protect his fame and his mother, and traveled to the Shapeless Island that holds those exiled by the gods in order to claim its head. He was granted five Noble Phantasms, probably the most ever granted to a single warrior in the history of Greece, by the gods, which include Harpe, winged sandals, a mirror shield, a helmet of invisibility, and Kibisis, a bag to contain the beast's head. He gathered as much information as possible, formed a strategy to use his equipment, and sought out to gain glory and become a true hero. He was completely fearless until arriving at the island, where just the nature of the island's barrier made him uncertain of victory despite having gifts from the gods. He quickly began to lose any feeling of glory, and simply wished to slay the creature as soon as possible so he could leave. He felt complete dread upon meeting the Gorgon, believing his promised glory to be useless and victory to be impossible even with the holy relics.
He used the mirror shield to keep from being petrified by the creature's eyes and the sandals to avoid its strikes, entering into a battle of attrition without any real chance of victory. He felt a true hero might be able to kill a creature that could be called a demon god, but felt someone who was still trying to prove himself had no chance. The only way for someone who still had human nature to defeat the creature was to allow it to encage itself in its own barrier by using Kibisis to reflect it back on its wielder. The beast, frozen upon suffering its own nightmare, was open to an attack from Harpe, and transformed back to the form of Medusa to allow her head to be taken.
Rider has short wavy dark red hair and wears a gray mask with black markings to conceal his face and his identity, along with a black cloak.
At school he has stylish hair and yellow green eyes, and wears the navy school uniform with a white shirt, orange tie, and gray pants.
While he is the only one of the seven summoned Servants to not have had a tragic ending in life, he now welcomes gory deaths after the form of the boy, who wished only peace for everyone, has buried itself in his heart and drives him on his insane killing sprees. He never wipes away his smile when showing his true face, but he knows his actions are the opposite of what his Master wished. Even still, he has decided that he will kill again and again for the sake of his Master's happiness, all while showing a face streaked with bloody tears that will not stop.
He attends Ayaka Sajyou's school and disguises himself as a normal high-school boy, showing his two-faced nature. He is the popular boy of his class due to having a modern sort of beauty, getting along well with others, and being gentle. He portrays himself as an overall good youth who is polite, emotional, and easily gets teary-eyed. His fault can be said to be that he tends to overindulge in the things that he likes to the point that he consumes them all at once.
Rider's Master is a 10-year-old boy, a victim of a "chimerization" experiment by the unknown antagonist of the last War. At the beginning of this War, he is entirely sustained by medical machinery. Rider hated the magus for being weak, but sympathized with his plight, and later became good friends with him. On the 7th day (the day the youth's life burned out), his Master used all his Command Spells, and made Rider fully incarnate, feeling that as Rider was someone who found happiness in life, his wish, the wish of a content man, will be a happy prayer to warm the heart of many. Moved, Rider fought alone for the one wish to resurrect his Master and grant him the happiness that a man should experience. His Master was the Third Master of the Holy Grail War and his Command Seal was located on the palm of his left hand, taking the form of a five-winged symbol.
With this, Rider dons a mask and disguises as a Master, joining the carnage that is the Holy Grail War.
Later, Caster sieges the school by throwing it into an alien realm. Both Rider and Ayaka are under assault, but because Rider is somehow connected to Caster he is able to save both of them by covertly dismantling Caster's alien realm.
Rider later faces off with Saber, after the priest informs Saber and Ayaka that there are some disturbances in the industrial area.
Perseus is the narrator for Rider's past. She claims that she disliked Perseus and the fact that he was the one who killed her, calling him a brat, and felt no sympathy for him even though it could be said he was also a victim of the gods. He later became a great hero, but she describes him as a "successful Shinji Matou."
Rider owns a wide selection of Noble Phantasms of varying ranks, but he has a limit that allows him to use only one at a time.
- Winged sandals - Given by Hermes, they are sandals capable of flight. They are quick enough to avoid the strikes of the Gorgon for a long period of time.
- Mantle - Called a helmet of invisibility in the legend, it changes and obscures the appearance of the wearer. It is likely descended from Hades' Cap of Invisibility, found within the Gate of Babylon.
- Mirror-shield - Given by Athena, it is a bronze shield polished to bear a mirror-like reflection that projects information from the surrounding environment on its surface, acting as sonar and radar so the user can rely on that information to move around without having to move himself away from the mirror. It can also be used as a sort of detection device as it transmits the heartbeats of those in the area. He did not use it to reflect Medusa's Mystic Eyes like in the legends, but instead focused only on the projected information to find her location and avoid having to look upon her.
- Kibisis: (鏡像結界の袋, Kibishisu?) - Used to defeat the Gorgon, it is the sack that held the demon's head. It is an anti-sealing Noble Phantasm that distorts the concept of the world through a mirror world-type Bounded Field that places the inside of the pouch on the outside and the outside of the pouch on the inside. It inflated and swallowed Perseus, creating a fold in space that placed the outside world into the inside of the bag, and Breaker Gorgon, which affects only what is inside, was reflected back, making Medusa look upon herself as if reflected in a mirror, allowing him to slay her with Harpe while she was immobilized.
- Bellerophon - It is the same as Medusa's Bellerophon, a Pegasus-handling bridle.
He most often uses the mantle and a mask to hide his true form when acting as a Servant. The mask acts as a fake Master that supplies him magical energy. In order to obtain the magical energy, he has to kill over and over in order to eat the souls of people. He is given a physical form by his Master while on the verge of death in order to challenge Saber.