The Eldian Restorationists were a radical group of Eldians in Liberio who plotted to destroy Marley from within using the Founding Titan, and restore Eldia as a world power. They rejected Marley's teachings and believed them to be lies. The symbol of the group was a cross-shaped scar on each member's body, which they make themselves with a knife.
They received weapons, funds, and historical documents from "the Owl," an informant from the Marley government. The documents were written in an unreadable language, but looking at illustrations, they concluded that Ymir Fritz had used the power of the Titans to cultivate lands, build roads, and bring wealth to mankind.
History: The remnants of the royal family that stayed in Marley formed a revolutionary army; however, this army was unsuccessful and was crushed. All members of this original organization, including Eren Kruger's father, were smoked out by Public Security Authorities and executed for treason against Marley. Grisha Yeager joined a similar anti-establishment group, organized by Kruger, at the age of 18 when a member of the organization came to his clinic and told him that they received information that his sister had been killed by a man in the Marley authorities.
Story: Kiyomi Azumabito reports to the Eldian military that Zeke Yeager had met with her in secret, describing himself as a true Eldian Restorationist for as long as he can remember. He regrets having to kill Eldians and devastate Paradis, though he explains that going along with Marley's plan was his best chance of uniting the Founding Titan with a Titan of royal blood. Despite allegedly holding his parents' ideology, he remarks that they were naive and would never have been able to restore glory to the Eldian Empire.
Source: Eldian Restorationists, available under CC BY-SA 3.0.