|I suspect their only purpose now is to put Zeke and Eren in contact with one another. Then, they'll remake the military with Eren at the top.|
|— Hange theorizes on the Yeagerists' plan|
The Yeagerists (イェーガー派 Yēgā-ha?) are a rebel faction supported by civilians and former soldiers from the Paradis military. Their goal is to reunite Eren Yeager with his half-brother Zeke, and to create the "New Eldian Empire" (新生エルディア帝国 Shinsei Erudia Teikoku?).
The Yeagerists believe Eldia is on the verge of destruction and only they can save the island by dedicating themselves to Eren Yeager. The rebellious faction claims those who subordinate themselves to the outdated ideals of the current Royal Government will only watch as they are devastated by the outside world.
Initially, the group was known to consist of over a hundred members, many of them from the Survey Corps. However, the true extent of their influence is unknown with many soldiers and civilians vowing to devote their hearts to Eren Yeager. Around 500 soldiers connected with the Yeagerists are present in Shiganshina District at the time of Marley's invasion. It is said the Yeagerists effectively control everything inside the Walls.
It is believed that the initial formation of the Yeagerists was around the time Zeke Yeager presented to Paradis Island the plan to rescue the Eldian people whose existence is threatened. This plan involves awakening the Titans sleeping within the Walls in a process known as "The Rumbling." 
Later on, Eren would begin to act on his own while defying the military that intended to betray Zeke Yeager by feeding him to Historia upon his arrival to Paradis. At some point, many people began to see Eren as their savior who could save the Eldian Empire.
The group steadily increases in size and waited for the right moment to strike. After Eren infiltrated Marley and the resulting Raid on Liberio, Eren was arrested for his actions and causing the deaths of several Survey Corps soldiers. Word spreads about his imprisonment and the four who leaked the story are arrested as well, while protesters demand that Eren be released.
Meanwhile, Yeagerists within the Survey Corps would begin covertly working with Zeke's trusted follower Yelena and procure wine for the soldiers detaining Zeke within the Titan Forest. This wine was taken off the captured Marleyan ships and is tainted with Zeke's spinal fluid.
Eventually, several members plant a bomb in a chair belonging to Commander Darius Zackly and place it in his office. They later trigger the bomb, killing Zackly and several members of the Military Police. At roughly the same time, Eren uses his newly granted powers of the War Hammer Titan to tunnel his way out of his cell and seals his escape route so he could not be followed. While this happens, the other members free Eren's allies and escape the prison as well. Regrouping out in the mountains, Eren meets with his followers and declares he will locate Zeke.
After Eren's escape, the top military officials meet with Dot Pixis who believes the Yeagerists will target anyone who knows where Zeke is being held and then capture Queen Historia, who could act as a replacement for Zeke. Pixis then declares they will negotiate with Eren using Zeke's location and grudgingly decides to disregard their assassination of Darius Zackly in order to avoid more deaths, stating it is a cheap price to pay. However, this is just a ruse to commandeer the Founding Titan from Eren. The Yeagerists would somehow receive word of the proposal, but it is rejected by Eren who refuses to entrust Commander Pixis with the fate of the island.
The Yeagerists later storm into a restaurant to attain the location of Zeke Yeager from Hange after receiving word from a Marleyan. The Yeagerists then capture those present at the restaurant including several officers of the Survey Corps and the Blouse family before departing to the Shiganshina District.
While in Shiganshina District, the Yeagerists interrupt a 109th Training Corps exercise in an attempt to seize control of the division. Floch is able to quickly drum up support from the sympathetic trainees, and rebuffs Keith Shadis, who he has the trainees beat to prove their loyalty to the Yeagerist cause. After praising the Yeagerist affiliated division, Floch once again demands that Hange take them to Zeke's location.
En route to where Zeke was being held, Floch and the Yeagerists accompanying Hange hear a loud rumble and wonder what it could be; following the sound, they discover a demolished wagon with a small Titan nearby. After Hange discovers the body of Levi Ackerman, the Yeagerists become startled when the Titan begins emitting steam and a fully healed Zeke Yeager emerges from the Titan's stomach; Hange then escapes with Levi's body, causing several Yeagerists to chase after her. At the same time in Shiganshina, Eren goes to the prison cell where a captive Gabi is kept, only to be ambushed by Pieck, a Warrior from Marley who infiltrated the Yeagerists.
Many armed soldiers join Yelena and Eren on the roof of the jail when Pieck supposedly prepares to divulge the location of Marleyan spies. Galliard, who had also been posing as a Yeagerist, transforms inside the compound and begins a battle with Eren. Concurrently, several airships fly over the district, parachuting in Marleyan soldiers who take the upper hand in a firefight with the Yeagerist forces.
As the Yeagerists scramble to fight the forces from Marley, the Beast Titan arrives and begins destroying the airships from atop Wall Maria. Eventually, the imprisoned Survey Corps soldiers are released and head towards the other imprisoned Eldians; after a brief scuffle, the others are freed as well and the two groups reluctantly join forces to fight Marley.
As the conflict continues, a squad of Yeagerists ambush the Cart Titan. After the Titan is seemingly defeated, Floch remains skeptical but is caught off guard and forced to retreat with the other Yeagerists when Marleyan soldiers open fire from the Cart Titan's carcass.