“For him, who wished for the happiness of everyone... ---- For his sake, I will kill you all.”
Rider (ライダー) is the Rider-class Servant of Isemi in the Second Holy Grail War of Fate/Prototype.
ProfileRider's True Name is Perseus (ペルセウス), 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.