Sieg was created by Yggdmillennia as one of their many homunculi suited to various purposes. Initially kept in a prana supply tank devised by Gordes Musik Yggdmillennia to grant energy for Servants, he was one of many allowed to be used by Caster of Black for usage as harvesting materials for his golems. Due to his unexpectedly high quality Magic Circuits, he was thought to be a good choice for Caster’s Golem Keter Malkuth.
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.