WARNING: The following contains spoilers for Attack on Titan: The Final Season, now streaming on Crunchyroll, Funimation and Hulu.
Ever since Mikasa Ackerman first spotted Eren Jaeger's Attack Titan during the Battle of Trost, it was clear he was something special. As Attack on Titan viewers found out about the existence of other Eldians who could change into Titans, the reality of what they actually are came to the forefront of the series' conflict.
Nevertheless, while there are many differences between these Titan Shifters and the so-called "Pure Titans" that menace humanity, the most apparent of these is their ability to control their own actions. Though this initially seems like a quirk in the Titan hierarchy, the distinction could be the key to either the destruction or salvation of the entire world.
The vast majority of Titans that appear in Attack on Titan are known as "Pure Titans." While the process by which Eldians become Pure Titans is somewhat obscured, it appears to require the spinal material of a previous Titan, either via consumption or injection. Once transformed into these hulking monstrosities, the individual loses all sense of humanity and becomes a mindless creature devoted to devouring people. Some slight exceptions to this exist, such as Connie's mother who retained rudimentary speech after transforming, but even these seem to lack any true sense of agency.
By contrast, the Shifters, also known as the Nine Titans, consist of Eldians with the ability to change back and forth between their human and Titan forms. Each of the Nine has a unique skill of its own, but they all possess the capacity to form complex thought and retain their own identity. Though new bearers of one of the Nine may initially be overcome with beastlike and destructive tendencies, this is always a temporary state brought on by unfamiliarity with their new powers.
Still, it's unclear why only the Nine Titans retain their cognitive faculties while the Pure Titans are forced to turn against humankind. Perhaps the most straightforward answer is that this lack of free will is what allows the Founding Titan, when combined with royal blood, to control them. This power was demonstrated on a small scale when Eren made contact with the Pure Titan of Dina Fritz, his father's first wife who was herself a descendent of the royal family.
At that time, Eren was able to command the other Pure Titans in the area to not only destroy Dina's Titan, but to fight against the Armored Titan, one of the Nine in the possession of Marleyan Warrior Reiner Braun. Notably, while Reiner was held back by the Pure Titans under Eren's control, he was not swayed by Eren's order. This implies that the Nine have some sort of resistance or immunity to the Founding Titan's domination. However, this minor display of the Founding Titan's abilities is nothing compared to its most recent use: the Rumbling.
In Attack on Titan's most recent episodes, a jaded Eren tricked his half-brother Zeke Jaeger to gain access to the Founding Titan's abilities once more. This time he did not use it to settle a single battle, but to unleash all the Colossus Titans within the Walls and kickstart the Rumbling, a plan to destroy the rest of the world by trampling them under foot. Yet apart from those encased in the Walls, Eren left the other Pure Titans to wreak havoc on Paradis Island. Additionally, in the wake of the Rumbling, Eren's army did not include the remaining members of the Nine Titans.
Though the Founding Titan can control Pure Titans, Eren may have purposefully ignored them so they could distract the Shifters, whom he could not control. This is corroborated by the fact that the Cart Titan, possessed by Marleyan Warrior Pieck Finger -- who by all accounts is Eren's enemy -- was still seen in operation in the midst of the Rumbling. Yet despite the Nine Titans' potential immunity being a flaw in Eren's plan, it may be an intentional part of the system.
During the events leading to the Rumbling, Attack on Titan revealed the Titans' origin, relating to the Founder Ymir. 2,000 years prior, Ymir was a young girl enslaved by the ancient Eldian King Fritz. Even after she came into contact with the lifeform that granted her the ability of the Titans, she submitted to Fritz's will, who subsequently used her to conquer innumerable civilizations. Upon her death, Fritz commanded the daughters he sired with Ymir to cannibalize her, resulting in Titan abilities being passed down to form the current state of the Eldian people.
While Eren used Ymir's desire to break free as a means of starting the Rumbling, this may be something Ymir herself accounted for. Though she created Pure Titans without free will to be commanded by the Founding Titan, she may also be the one ensuring the Nine Titans' independence. As the overseer of the Paths that connect all Eldians, Ymir exists outside of linear time, and can likely see the future just like the Attack Titan.
Despite being part of Eren's plan now, she may have foreseen these events long ago and created a contingency plan for the apocalyptic situation of the Rumbling. Enabled by Ymir's gift of free will, it may now be up to the remaining members of the Nine to stop Eren -- the fate of Attack on Titan's entire world depends on it.
