“…For him, who wished for the happiness of everyone—For his sake, I will slaughter you all.”and “The Holy Grail is excessive for the tastes of the magi. As a Heroic Spirit, I will use it correctly, the way it was meant to be used.”
A Stray Servant with no Master. His true name is the hero from Greek Mythology, Perseus. Though he is the only one of the seven summoned Servants to not have a “tragic end”, he welcomes gory deaths even as the form of the boy, who wished only peace for everyone, buries itself in his heart and drives him on his insane killing sprees. He owns a wide selection of Noble Phantasms, like the Sandals that fly through the air, the Mantle that changes the appearance of the wearer (a helmet in the myths), the bronze mirror shield that Athena gave him (equipped with radar and sonar), the snake hunting sickle Harpe (kills immortals), the bag that stores the head of monsters Kibisis (a mirror world type bounded field), and the familiar Pegasus-handling bridle from FSN, Bellerophon. He is the very person who killed FSN Rider (Medusa).
Armor and Mask: When acting as a Servant, Rider hides his true form behind a mask and a cape. This cape is a Noble Phantasm that can obscure the appearance of its wearer. Though Rider holds a large variety of Noble Phantasms, he has a limit that allows him to use only one at a time.
The Mask acts as a fake Master, supplying him with magical energy. In order to get enough magical energy, Rider has to kill again and again (most likely to eat souls so he can get power). His Master, on the verge of death, gave him a physical form, which he used to challenge Saber, but…
Usually, he disguises himself as a normal high-school boy in Rider’s class. He’s the popular boy of the class due to having a modern sort of beauty, getting along well with others, and being gentle. The scenes of him at school show his two-faced nature.
When showing his true face, Rider never wipes away his smile. In his heart, he knows this is the opposite of what the boy wanted, but for the sake of his Master’s happiness, he would kill again and again. What sort of expression is he showing on the face streaked with bloody tears that will not stop?