| The subject of this article is anonymous.|
The name used is an alias, nickname or surname found in an official source, but the actual in-universe name of this person is unknown in this continuity.
|War Hammer Titans|
|◄ Preceded by|| Younger sister of Willy||Followed by ►|
|Unknown|| Eren Yeager|
|Brother. You have fulfilled your...duty...as a Tybur.|
|— The younger sister of Willy commemorates her brother's legacy|
The younger sister of Willy was a lady of inconspicuous appearance, standing relatively tall with dark eyes. Her hair was dark and slightly wavy, kept behind her head in a bun, and her facial features were small and gaunt. She wore a black and white dress of modest design.
As the War Hammer Titan, the younger sister of Willy possessed a muscular masculine body standing at a height comparable to the Attack Titan. The head of her Titan was covered by what appears to be a second layer of skin shaped like a cowl covering its ears and much of its face, with only its eyes, mouth, and jaw exposed through a segmented mesh of skin. Her Titan carried a massive war hammer of hardened Titan flesh standing at around twice the height of its body.
At some point within the past thirteen years, she inherited the power of the War Hammer Titan, though her inheritance of this power was kept a closely guarded secret. Upon inheriting the memories of her predecessors, she shared her knowledge with her brother Willy, who was stunned by the revelations she inherited.
When the Tybur family arrives in Liberio following the Marley Mid-East War, the younger sister of Willy joins her family to take up residence within the headquarters. After Willy introduces Magath to the family, he asked Magath if he was able to identify the user of the War Hammer Titan. Magath admits he has no clue but imagines the leader of the headquarters is one of the few who are aware of the War Hammer's identity.
The younger sister of Willy observes Willy's production from backstage during the night of the festival, where he shares the knowledge of the Great Titan War which he learned. Suddenly, the production comes to a halt when the Attack Titan bursts forth from the building behind the stage, killing Willy and starting a rampage. The younger sister of Willy survives the destruction of the stage and honors her brother for his work, starting her transformation into the War Hammer Titan. However, the Attack Titan turns to assault her before her transformation can complete, relentlessly beating her Titan with hardened fists. She eventually regains the advantage, using the power of her Titan to impale the Attacker on a massive pike of hardened Titan flesh while she regenerates her body.
Upon fully regenerating her Titan, the younger sister of Willy generates her Titan's signature war hammer to finish off the Attack Titan. It evades her first swing but is caught in a trap of spikes created by the War Hammer. Without any desire to consume the Attack Titan to gain the stolen Founder, Tybur decapitates the Attack Titan, exposing Eren Yeager, usurper of the Founding Titan, within the nape. She asks Eren if he has any final words, and in that moment he calls out for Mikasa to strike. Unaware of the counterattack, eight explosive rods are shot into the War Hammer's nape, detonating as Mikasa maneuvers around the Titan. The younger sister of Willy's War Hammer collapses from the devastating blow.
Despite the complete destruction of the War Hammer's nape, the younger sister of Willy is able to construct a crossbow, and fires a bolt towards the Attack Titan's nape. The strike is barely dodged, and forces Eren to abandon his Titan's body. The younger sister of Willy engages Mikasa, who continues to barrage her Titan with Thunder Spears, though to little effect.
Meanwhile, after realizing that the true location of the younger sister of Willy's body is behind and underneath the ruins of the stage, Eren transforms again and grabs the crystal encapsulating her body. After snapping the cable of flesh connecting the younger sister of Willy to her Titan, the War Hammer collapses. Eren goes to devour the crystal, though the attack of the Jaw and Cart Titans are able to distract Eren while a new cable is made, connecting to the ground. Eren is given only a few moments realization before his Titan is impaled by numerous pikes, stemming from the War Hammer's new body.
Having spent all her energy in the last attack, the younger sister of Willy finds herself unable to pierce the Attack Titan's nape through its hardened forearm. Mikasa fires a Thunder Spear at her crystal, yet fails to even crack it. Later, the Attack Titan attempts to bite the crystal again, but is unsuccessful and breaks its teeth on the hardened surface. Declaring that the War Hammer is out of moves, Eren exits his Titan's nape and transforms again, picking up the crystal in his new Titan body.
The younger sister of Willy is seen observing the battle between both the Attack and Jaw Titans; she is visibly surprised when one of the strikes from the Jaw Titan causes some damage to the crystal surrounding her. The Attack Titan takes note of this as well and devises a plan to exploit this newfound revelation. After disabling the Jaw Titan, the Attack Titan jams the crystal containing the younger sister of Willy into the mouth of the Jaw Titan and begins to squeeze. Both the younger sister of Willy and Galliard realize what the Attack Titan is doing but they are powerless to stop it; the younger sister of Willy silently acknowledges her fate as the crystal around her fragments and shatters, ripping her to shreds and causing her remains to be eaten by the Attack Titan.
- Structural hardening: The signature ability of the younger sister of Willy's War Hammer Titan was the power to effortlessly generate massive spike-like structures out of hardened Titan flesh, ranging from ground spikes small enough to go unnoticed by other Titan opponents or massive pikes large enough to impale the 15-meter Attack Titan far above the ground. This structural hardening also allowed for the War Hammer to create objects such as weapons, notably the signature war hammer for which her Titan is named. Much like the Female Titan, the younger sister of Willy could use this ability to encase herself in a crystalline barrier, though she retains consciousness in order to be able to control her Titan body.
- Regeneration: Like all others with the power of the Titans, the younger sister of Willy was able to regenerate any wounds received in battle. After receiving a devastating barrage of punches from the Attack Titan with no defensive measures, she was able to fully regenerate her Titan's missing flesh and limbs in a matter of minutes.
- Remote operation: While encased in crystal, the younger sister of Willy could create a body of a Titan to be remotely controlled from a distance, enabling her to regenerate her entire Titan body without sustaining any damage to her human form, as well as circumventing the typical weakness of the Titan's nape. The crystal and the Titan body were connected to her through a long cable of flesh. When this ability was utilized, she generated the body of the War Hammer Titan from the feet upwards rather than from the nape outwards.
Her only weakness appeared to be low levels of stamina. She was able to fully transform only once, and then was able to manage a second (although partial) transformation during her battle against Eren and the Survey Corps before running out of energy.