Yeagerists

"It is likely their objective is to reunite Eren Yeager and Zeke Yeager..... and purge those who would oppose making Eren the leader."

- Hange theorizes the plan of action by the Yeagerists

The Yeagerists are a rebel group consisting of former soldiers from the Paradis military. Their presumed goal is to reunite Eren Yeager with his half-brother Zeke and to possibly take control of the nation of Paradis.

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 the nation of Paradis 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 summon 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 demons. When the military decided on a more passive route, this would cause many to turn their backs on the government and see Eren as their savior who was willing to do what needed to be done to ensure victory.

The group would steadily increase in size and wait for the right moment to strike. After Eren infiltrated Marley and the resulting Battle of Liberio, Eren was arrested for his actions and causing the deaths of several Survey Corps soldiers. Word would spread about his imprisonment and the four who leaked the story were arrested as well, while protesters would demand his release. Eventually, several members planned a coordinated bombing on the central military headquarters. Placing a bomb in a chair of Commander Darius Zackly, three members went under cover as part of the Survey Corps and placed it in his office. They would 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 met with his followers and declared they need to find Zeke as soon as possible.