Sieg is a homunculus created in the Great Holy Grail War of Fate/Apocrypha as a mass-produced vessel for Heroic Spirits. After Siegfried sacrifices himself to save him, Sieg inherits the dragon-slaying hero's heart and body, gaining immense power including flight and Balmung. Pursuing his own life and freedom, he travels with Astolfo and later meets Jeanne d'Arc, whom he deeply loves. Despite his artificial origins, Sieg yearns for genuine human experiences and fights against oppression, embodying themes of identity and autonomy in the Fate series.