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Seibzehn is a titanic blue Gear controlled by Maria Balthasar, "Siebzehn" is German for "Seventeen", perhaps inferring that Seibzehn was the seventeenth version of its design. Seibzehn is unlike most other Gears in Xenogears in a variety of ways, one of which being that Maria does not pilot it by sitting in a cockpit inside it—she rides on its head and controls it with verbal commands, similar to Giant Robo.
It was created in Solaris by master Gear designer Nikolai Balthasar at the horrific cost of hundreds of land dwellers' lives, including Claudia Balthasar. Because part of the Gear's neural make up is comprised of Wels, the Gear itself has a personality and a degree of autonomy. Nikolai programmed it to escape Solaris and flee to Shevat carrying young Maria with it. Ever since Maria has used it to defend Shevat from any and all threats to its safety.
When Shevat is attacked by a brainwashed Nikolai who had also been merged into his next generation Gear Achtzehn (eighteen in German), Maria reluctantly rode Seibzehn out to confront her father. Nikolai was able to regain control of himself but couldn't live being trapped in the shell a Gear so he activated one of Seibzehn's dormant weapons, the Gravitation Cannon. To her horror, Seibzehn acted beyond her control and fired the cannon on Achtzehn and completely obliterated it and Nikolai.
In battle, aside from Xenogears, Seibzehn is the strongest Gear in the game. Its physical attacks are arguably the best of all the "normal" Gears in the game, but its power is offset by low speed and sometimes poor accuracy, much in the same vein as Rico's Stier. Seibzehn can also appear while Maria is on foot through her "Call" special ability, and its concept and physical design are based on Giant Robo of super robot fame. Maria can also summon Seibzehn via Controls ability.
In contrast to Maria when she attacks them in a weak manner and summoning Seibzehn to attack via controls, Seibzehn has more movements than the pilot Maria herself. It can shoot both of its fists similar to a rocket and more focus on beating opponents in a close range and also shoots finger missiles on the Gear's fingers. In the boss battle against Fei and the group at the top of Babel Tower, Seibzehn's moves are retained as one of the Gear Deathblows are used to deal heavy damage. In one of its Infinite Deathblows Maria Beat, Maria wields a red heavy mallet to directly hit the opponent's head to deal massive damage.
- Mega Hammer: Seibzehn does a heavy punch on its opponent.
- Dyna Fight: Seibzehn does a punch and a heavy blow followed by shooting finger missiles on its downed opponent.
- Gravity Fist: Seibzehn tackles its opponent and lands a straight punch followed by two rocket fists.
- Dyna Rush: Seibzehn does two heavy punches and a knee to its opponent.
- Head Dive: Seibzehn flies and dives to make a downward headbutt on its ground opponent to deal heavy damage.
- Gravity Press: Seibzehn lifts and throws to its opponent and heavy stomps it to the ground dealing heavy damage.
- Meteor Press: Seibzehn flies and shoots rocket fists on its ground opponent and piles the opponent down to the ground.
- Iron Break: Seibzehn lifts and throws the opponent into the ground and lands heavy stomp on its downed opponent.
- Iron Storm: Seibzehn does a heavy punch followed by a heavy blow and another heavy punch and a knee and flies to shoot its downed opponent with two rocket fists.
- Fire Drive: Seibzehn does a heavy punch twice with fire embedded in on its fists creating multiple fire damage on its opponent.
- Magneto: Seibzehn lifts and throws its opponent in a rainbow flash of light and heavily stomps on its downed opponent multiple times and shoots rainbow flames at the rocket on its back.
- Maria Beat: Seibzehn does multiple punches followed by a heavy blow as Maria wields a heavy red mallet to directly hit the opponent's head in a massive damage.
- At one point in Xenogears development, Maria was meant to have an Omnigear just like the other main characters called El-Seibzehn. Unfortunately, due to time and budget constraints, the portion of the game where Seibzehn is aligned with an Anima Relic was cut out of the game and El-Seibzehn went with it.
- In Xenosaga Episode III: Also sprach Zarathustra, Mai Magus fights with an Auto-Tech called Leupold that strongly resembles Seibzehn.