« For him, who wished for the happiness of everyone... ---- For his sake, I will kill you all. »
Rider's True Name 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 sacred 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.
"I don’t have a wish for the Grail.
As this body had already been filled.
I always believed.
In the world’s warmth.
In the goodness of people."
Perseus says that because he was always blessed, and that's why when he saw Aro's figure, he knew that he needed to save Aro from himself.
He felt deep sympathy for Aro, due to his illness since he was bound to a hospital bed and therefore, he could not witness the outside world, Perseus decided that he will wish to heal Aro to see the Perseus constellation under the starry night sky.
Perseus mentioned that he didn't actually feel alive, so it didn't suit the pain that Aro felt.
When Aro dies, Perseus becomes fuelled by anger.
"I remember my anger till it was repulsive, at everything that couldn’t save you.
――――My anger melted muddily, with my honour, with my affection. " 
Although he has sociable manners, is easily moved, and is a gentle hearted young man, he has the flaw that, once motivated to do something, he will not stop until he gives it his all and then some more. In this sense, he is also an anomaly. Once he decided to stain his hands with wrongdoing, he became capable of killing innocents with a gentle manner and smile.
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 Medusa's past in Fate/hollow ataraxia. After Medusa tells the story of her death, she mentions that having her head used so casually by "an...adolescent..." angers her greatly. Shirou asks her to clarify, "You mean Perseus, right...?" and makes the point that he "can't call him a "victim," but it does sound like he got tricked into it." Medusa replies "I suppose so." and agrees that he was fooled by the gods.
While she does mention that in any case, she saw him as a "mere mosquito" at the time, and that she does not feel any sympathy for him, Medusa does give him credit; "Personally, there is no way I could bring myself to like him. Nevertheless, he did become a fine character afterwards." However right after she said this, when Shirou asks what Perseus was like, she insults him by calling him a "successful Shinji."
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
- 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.
- ↑ Fate Prototype Fragments of Blue and Silver - Fan Translation by PhoenixRising - Fate/Prototype Beautiful Mind Act 5 - Part 3 http://forums.nrvnqsr.com/showthread.php/5621-Fate-Prototype-Fragments-of-Blue-and-Silver-Fan-Translation
- ↑ Prototype material
- ↑ 3.0 3.1
- ↑ Fate/Hollow Ataraxia - 10/08 - Mirror of Kibisis