Yeagerists

"Their one and only purpose for now must be to put Zeke and Eren in contact with one another. Then, they'll purge the Corps with Eren at their center."

- Hange theorizes on the Yeagerists' plan

The Yeagerists (イェーガー派) are a rebel group consisting primarily of former soldiers from the Paradis military, mostly from the Survey Corps. 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).

Duty
After Eren Yeager was revealed to be in possession of the Founding Titan, many called on him to use his abilities to fight against the nation of Marley who were behind the failed Paradis Island Operation that took place over the course of several years. Preferring to use military actions over diplomacy in dealing with enemy nations, they renounced the path set forth by the Eldian military and swore allegiance to Eren. Seeing him as a proper leader, they agreed with the "Rumbling" plan set forth by Eren and his older half-brother Zeke and vowed to make it come to pass, regardless of the cost.

Structure
The group consists of at least a hundred members, many of them from the Survey Corps.

Marley arc
It is believed that the initial formation of the Yeagerists was around the time when Zeke Yeager presented to Paradis Island the plan to wipe out the nation of Marley known simply as "The Rumbling." When Eren confirmed that he had the ability of the Founding Titan, which could control an army of Colossus Titans, many saw this as a perfect opportunity to wipe out the nations that still viewed the island (and the inhabitants) as devils. When the military decides on a more passive route, this causes many to turn their backs on the government and see Eren as their savior who is willing to do what needs to be done to ensure victory.

The group steadily increases in size and waits for the right moment to strike. After Eren infiltrated Marley and the resulting Battle of Liberio, Eren is 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. Eventually, several members plan a coordinated bombing on the central military headquarters.

Placing a bomb in a chair of Commander Darius Zackly, three members went undercover as part of the Survey Corps and placed it in his office. They later trigger the bomb, killing Zackly and several members of the Military Police; at roughly the same time, Eren used his newly granted powers of the War Hammer Titan to tunnel his way out of his cell and sealed his escape route so he could not be followed. While this happened, the other members freed Eren's allies and escaped 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.