Lancer of Red is the Lancer-class Servant of Shirou Kotomine of the Red Faction in the Great Holy Grail War of Fate/Apocrypha. Originally summoned by one of the five puppet Masters of the Red Faction, he later becomes one of the Servants of Shirou.
Lancer's identity is Karna, Son of the Sun God, the invulnerable hero of the Indian epic Mahabharata. Karna was born between a human mother Kunti and the Sun God Surya. As proof of being the son of Surya, he was given a golden armor that granted invulnerability. Karna was born to become a great hero. However, Kunti abandoned Karna and became the queen of King Kuru. Not knowing who his mother was, Karna grew up with a low social status. However, he soon made his existence known to the world by becoming the honored guest of the Royal House Kauravas, who were hostile to King Kuru. Karna participated in the wars around the borders and fought on an equal footing with Arjuna, son of Kunti and Thunder God Indra. Karna eventually fell in battle after being robbed of his golden armor by Indra, suffering many curses, and losing all of his allies. He died in the hands of his half-brother Arjuna. However, he did not hate anyone and accepted his death.
(Source: Type-Moon Wikia)