The Jaegerists (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 Jaeger with his half-brother Zeke, and to create the "New Eldian Empire" (新生エルディア帝国 Shinsei Erudia Teikoku?). In the month following the Raid on Liberio, the Jaegerists conducted a military coup and took power within the Walls.
The Jaegerists believe Eldia is on the verge of destruction and only they can save the island by dedicating themselves to Eren Jaeger. The rebellious faction claims those who subordinate themselves to the outdated ideals of the current Royal Government will only watch as they are devastated by the outside world. 
The group is known to consist of over a hundred members, many of them from the Scout Regiment. However, the true extent of their influence is unknown with many soldiers and civilians vowing to devote their hearts to Eren Jaeger. It is said the Jaegerists effectively control everything inside the Walls.
After Eren's infiltration of Marley and the resulting Raid on Liberio, he was arrested for his actions which resulted in the deaths of several Scout Regiment soldiers. Word starts to spread about his imprisonment and the ones who leaked the story are arrested as well. Meanwhile, protesters start to gather and demanding that Eren be released from his confinement.
Meanwhile, Jaegerists within the Scout Regiment would begin covertly working with Zeke's trusted follower Yelena and procure wine for the soldiers detaining Zeke within the Forest of Giant Trees. This wine was taken off the captured Marleyan ships and is tainted with Zeke's spinal fluid; it was also instructed to be given out to many senior heads of the Military to neutralize them as possible threats.
Eventually, several members plant a bomb in a chair belonging to Commander Dhalis Zachary and place it in his office. They later trigger the bomb, killing Zachary and several members of the Military Police. At roughly the same time, Eren uses his newly granted powers of the War Hammer Titan to escape his cell. 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 orders them to locate Zeke.
After Eren's escape, the top military officials meet with Dot Pyxis who believes the Jaegerists will target anyone who knows where Zeke is being held and then capture Queen Historia, who could act as a replacement for Zeke. Pyxis then declares they will negotiate with Eren using Zeke's location and grudgingly decides to disregard their assassination of Dhalis Zachary in order to avoid more deaths, stating it is a cheap price to pay. However, this is just a ruse of the commander to abduct their leader and steal the Founding Titan from him. The Jaegerists would somehow receive word of the proposal, but it is rejected by Eren who refuses to entrust Commander Pyxis with the fate of the island.
The Jaegerists later storm into a restaurant to attain the location of Zeke Jaeger from Hange after receiving word from a Marleyan. The Jaegerists then capture those present at the restaurant including several officers of the Scout Regiment and the Braus family before departing to the Shiganshina District.
While in Shiganshina District, the Jaegerists interrupt a 109th Cadet 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 Sadies, who he has the cadets beat to prove their loyalty to the Jaegerist cause. After praising the Jaegerist 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 Jaegerists accompanying Hange hear a loud rumble and wonder what it could be.