Siegfried is an infantry hero available in the Scenario Editor in Age of Empires II. He is based on Sigurd, the legendary hero of Norse mythology, as well as the central character across several Scandinavian sagas and by extension, Wagner's opera of the same name from his Ring Cycle.
In-game, he is represented by a Champion. Like other infantry, Siegfried is affected by all upgrades that affect infantry as well as any other bonuses that affect them. As a hero, he cannot be converted and can regenerate health (since The Conquerors).