Sieg "Homunculus"
About
Sieg comes into consciousness without anything to form a personality. He is scared of everything but himself, believing to be nothing. He calls himself transparent and colorless because he possesses nothing and had received nothing from his birth.
Sometime later, he escapes from the Yggdmillennia castle in the middle of a battle with the Red Faction with some help from Astolfo, who inspires him to make a life for himself. During the escape, he is gravely injured and only survives when Siegfried transplants his heart into him, sacrificing his life in the process.
With Ruler's help, he takes refuge in a farmer's house but decides to return to the Grail War to try and free his homunculi brethren.
(Source: Type-Moon Wikia)
