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Grice was a member of the Eldia Restorationists, a secret organization which had the goal to set the Eldians free from Marley. Along with his fellows, he was arrested and sent to Paradis island after Zeke Yeager reported them. Although he was supposed to be turned into a mindless Titan, he was instead left in his human form to be chased by his comrades.
All Eldian Restorationists sent to Paradis had their families implicated as well. Therefore, Colt was forced to join the Marley military in order to prove the rest of his family's innocence. Colt's younger brother, Falco, did the same to also protect his family. When Colt was confirmed to inherit the Beast Titan, peace was returned to the Grice family.
During the Fort Slava battle, Falco and Colt are among the Eldian soldiers fighting for Marley. Falco gets a head injury and forgets what is happening around him but is found and rescued by his brother.
At the end of the war, all the Eldians can go back home and the brothers see their parents.