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|The Jaw Titan is an assault unit. Its small size makes it the swiftest of all. Its powerful nails and jaws can crush just about anything.|
|— A Marleyan analysis of Marcel's Jaw Titan|
Those who hold the power of the Jaw Titan have demonstrated significant amounts of speed and agility in their Titan form. Though the Jaw Titan is relatively small and weak, its mobility excels in terrain such as Titan Forests. The Jaw Titan also possesses, as its name implies, an incredible biting strength which can be used with devastating efficiency against both Titans and man-made structures.
Porco Galliard's Titan appears to have armored skin on the fingers, and the jaw of his Titan form is much more thick and cog-like than the Titan held by Ymir.
The Jaw Titan, like all the Nine Titans, was brought into the world after the death of Ymir Fritz. In the 1,700 years following her death, the Jaw Titan was held in the possession of one or several of the warring Eldian houses subservient to the Founding Titan. After these seventeen centuries when the 145th King of the Fritz family abandoned the conflicts of Eldia and relocated to Paradis, the Jaw Titan was taken by the nation of Marley during the Great Titan War.
Around the year 843, Marcel Galliard was chosen to inherit the power of the Jaw Titan. He would later make use of his Titan form during a war between Marley and an enemy nation, using his Jaw Titan to destroy the enemy's artillery guns.
In the year 845, the nation of Marley sent out Marcel, Reiner Braun, Bertolt Hoover, and Annie Leonhart across the ocean to the island of Paradis as the start of the Paradis Island Operation. However, at some point before the attack on Shiganshina District, a Titan approached Reiner and nearly had him in its grasp until Marcel moved him away to safety. Marcel was caught in Reiner's place, and he was eaten alive as his comrades escaped. The Titan inherited Marcel's Jaw power and regained its human form: an Eldian girl named Ymir, who had been wandering as a Titan for around sixty years.
Choosing to start a new life for herself, Ymir made her way into humanity's territory and began living as a thief in the interior territories of the Walls. At this time, she overheard the story of "Krista Lenz" from members of the Church of the Walls and her plans to join the 104th Training Corps. Desiring to meet her for political and personal reasons, Ymir joined the Training Corps in the same year as Krista.
At some point during their training, Ymir and Krista face difficulties escorting a sick trainee to shelter in harsh winter conditions. Wanting to see Krista survive, she sends her off to the shelter alone and uses the Jaw Titan to bring their fellow trainee to safety at the base of a cliff side. Krista wondered at how Ymir could have possibly reached the shelter before her. Ymir agreed to tell her secret to Krista under the condition that when the secret of her Titan powers are publicly known, Krista would go back to living under her true name.
During the Wall Rose invasion in the year 850, Ymir is among the recruits of the Survey Corps when they are cornered by Titans at Utgard Castle. Seeing no hope for escape, Ymir chooses to make use of her Titan form to confront the Titans. Transforming before her friends' eyes, she battles the Titans at the base of the castle as the others comprehend the revelation of her powers. Reiner and Bertolt recognize her Titan as the one that ate Marcel five years ago and determine that they must capture her along with Eren when the time comes. When Ymir begins to struggle, Krista offers her encouragement. Ymir's Jaw Titan ascends the castle and saves the others from its collapse. When the dust settles, Ymir's fatigued Titan is unable to continue fighting against the Titans, but her life is saved with the arrival of the Survey Corps soldiers from the Stohess District raid. She is safely extracted from her Titan, and Krista reveals her true name as Historia Reiss.
Shortly afterwards, when Bertolt and Reiner reveal their identities as the Colossus and Armored Titans, Ymir is taken by the two warriors along with Eren. After some questioning, Ymir agrees to cooperate with them in order to capture Historia. When the Survey Corps finds their location, she takes Historia into her Titan's mouth and escapes with Bertolt and the Armored Titan. Ymir remains within her Titan form throughout the escape, taking a moment early on to partially expose her human form in the nape in order to speak with Historia. Ymir abandons the Armored Titan when it is swarmed by a horde of Titans lured by Commander Erwin Smith and assists the Survey Corps for a short time, but upon seeing Eren's use of the Founding Titan during the battle Ymir chooses to go with Reiner and Bertolt rather than staying with the Eldians of the Walls. After the battle, Ymir escapes with Bertolt and the Armored Titan to the ruins of Shiganshina District.
At the climax of the Marley Mid-East War during the battle of Fort Slava, Galliard is present in the trenches dug by the Marley warrior unit surrounding the fort. Cadet Colt Grice suggests making use of Galliard's Jaw Titan and Pieck's Cart Titan against the defenses of the Mid-East Allied Forces, but Commander Magath rejects the proposal due to the presence of anti-Titan artillery patrolling the perimeter of the fortress. However, once cadet Gabi destroys the armored train carrying the artillery using explosives, Galliard is sent out onto the battlefield. His Titan proves to be more than enough to defend against the turret-fire from the bunkers surrounding Fort Slava, and Galliard's Titan makes quick work of Marley's enemies.
Galliard's attack on the Mid-East defenses is met with support from the 800-warrior unit and Pieck's Cart Titan in full battle armor. Using the immense strength of his Titan's jaws, Galliard rips the railways from the ground, ensuring a complete immobilization of the Allies' defenses. Shortly afterwards, Galliard retreats as Zeke Yeager begins a Titan airstrike directly above the fortress, dropping dozens of mindless Titans onto the Allies. When Reiner Braun joins the fight as the Armored Titan, Galliard assists him in destroying the remaining anti-Titan artillery stationed within the ruined fortress.