Rider of "Red" is the Rider-class Servant of Cabik Pentel of the Red Faction in the Great Holy Grail War of Fate/Apocrypha. Shirou Kotomine assumes command over him before he even meets his Master, and through his machinations he later officially becomes Rider's Master.

Spoiler

Rider's True Name is Achilles (アキレウス, Akireusu), one of the famous heroes in the Trojan Wars. Born from the sea goddess Thetis and the hero Peleus, Achilles was blessed by the gods from birth. His mother Thetis, having greatly loved her son, had warmed him within holy flames in order to try and make him immortal. But her husband Peleus had opposed it, saying “Then that would destroy Achilles as a human”, and in the end Achilles grew up while only a certain part of him remained human. Eventually, when war broke out between Troia and Achaea, Achilles was asked the following by his mother Thetis: "Do you wish to live a long and peaceful life without having your name recognized by the masses? Or do you wish to live the short life of a hero with brilliant achievements in battle?" Achilles choice went without saying. At the same time as his mother felt pride in his decision, she also felt heartbreakingly sad, because his fate had been predetermined since he was born. If he lived as a hero, he would have a short life, as if sprinting through it at full speed.