Berserker of Red is the Berserker-class Servant of the Red Faction in the Great Holy Grail War of Fate/Apocrypha. Originally summoned by Deimlet Pentel of the Red Faction.
Berserker's true name is Spartacus (スパルタクス), the strongest Gladiator and the ringleader who raised a large-scale rebellion of slaves in Ancient Rome known as the War of Spartacus.

Spartacus was a Thracian gladiator captured by Romans who became a symbol of rebellion. He was affiliated with a gladiator training camp in the Campania region, but escaped with seventy-eight of his comrades in 73 B.C. He managed to repulse an assault force of nearly three thousand dispatched by the Romans, leading to him being known as a hero who inspired armed uprisings by slaves in many places. His army grew to seven thousand men, and they kept defeating the Roman army without losing a single battle. They were unable to cross the Alps and instead decided to head to the island of Sicily. They hired pirates to make the voyage, but were betrayed and attacked by the Roman Legions. He always fought at the head of the front lines, and even when the revolt was subjugated by the army, he fought until his death. Perhaps due to that, Spartacus' corpse is said to have been torn to pieces and never found anywhere on the battlefield afterwards. It is believed his body was scarred beyond recognition after he fell in that battle.

Being his first and only loss, he remained a shining beacon of hope to the under trodden slaves. His name was carved in history as a hope of the oppressed humans.

Unlike the rest who eventually become the Servants of Shirou Kotomine, he contracts with Caster of Black when captured by the Black Faction.
Type Moon Wikia