Developed in secret with funding from Prince Schneizel el Britannia, the Siegfried is part of the clandestine research project Code R. Originally funded by Schneizel's late brother Clovis, Code R is dedicated to the research of individuals linked to the mysterious power known as Geass. The massive Siegfried, classified as a "Knight Giga Fortress" rather than a Knightmare Frame, makes use of both the latest Knightmare technology as well as data from Code R. In keeping with its altered title, the Siegfried bears little resemblance to normal Knightmare Frames, consisting of little more than a large, flying cockpit. Its size is comparable to that of the Gawain, another larger than normal Knightmare. A Nintendo DS version of the series featured a pink version of this unit referred to as the Siegfried Kai, which is piloted by Euphemia li Britannia.
The Siegfried has a unique cockpit system wherein the pilot's nervous system is connected directly to the Knightmare, allowing for complete mental control. The cockpit is also considerably larger, allowing the pilot to move freely inside, though there are no monitors and no controls to speak of aside from the interface itself. The neural interface allows the pilot to react much quicker in battle, such that they can dodge attacks that would be nearly impossible for an ordinary pilot. However, the interface can only be used by those with nerve potential beyond that of ordinary humans, limiting the potential operators to V.V. and Jeremiah Gottwald, the latter having been modified by Code R to replicate C.C.'s abilities.
The Siegfried is equipped with a Float System and has a number of verniers mounted on its frame, giving it excellent mobility despite its size. Its main armament consists of a set of Slash Harkens, which consist of the five conical structures on its frame and two ordinary versions mounted on the front. The five conical Slash Harkens are nearly as large as an ordinary Knightmare. It also possesses generators which can electrify the hull to keep enemies at bay. Using the conical Slash Harkens, it can perform a devastating attack in which it spins like a top to obliterate anything in its path or act as a physical barrier against projectiles. Its tough construction and energy shields make it incredibly durable; it manages to survive having a building dropped on it without even taking cosmetic damage, and likewise survives water pressure at depths that crush the Gawain.[31]
The Siegfried is stolen by Jeremiah Gottwald during the first battle of Tokyo when he goes into a psychotic rage upon hearing the voice of the terrorist leader, Zero[30] Unable to defeat it, C.C. ultimately resorts to using the Gawain in a kamikaze attack, wherein she ensnares the Siegfried with her Knightmare's Slash Harkens and drags them both into the ocean depths. Jeremiah and the Siegfried somehow survive and are next seen with the Geass Order. V.V. pilots it against Lelouch during the assault on the Order, but it is destroyed by the combined efforts of Princess Cornelia and Lelouch.
Following its destruction, the Siegfried is rebuilt by Rakshata into a new Knightmare for Jeremiah known as the Sutherland Sieg. Its size and basic shape are similar to the original Siegfried, but the weapons placement, as well as its armament in general, is much different. A large cannon is mounted on the underbelly, and it is equipped with an array of missile launchers. The Britannian energy shielding is replaced with the radiant wave surger-based shielding used on the Guren. The five conical Slash Harkens are also positioned differently. Two are mounted on the front as makeshift hands, and can channel the energy from the generators to stun opponents. The other three are located on the aft side: two are lined up parallel with the engine, and the remaining one is placed perpendicular to the cannon. It retains the neural interface from the Siegfried, which Rakshata finds disappointing since the technology isn't feasible for mass-production. As the name implies, the Sutherland Sieg is built around a Siegfried-colored Sutherland, the head of which sticks out of the top normally. The Sutherland has Float System attachments equipped to its feet in place of Landspinners, allowing it to fly. The Sutherland Sieg's main body is destroyed by Anya Alstreim's Mordred, after which the Sutherland within breaks away and is self-destructed by Jeremiah to damage the Mordred.
