At a young age, Marcel and Porco joined the Marleyan military to become Warriors. They were both certified Warrior candidates and were shortlisted to receive the Power of the Titans. Marcel was a candidate for the Jaw Titan and inherited it from his predecessor, while Porco was a candidate for the Armored Titan but Reiner Braun obtained it rather than him. This was due to Marcel's intercession, who managed to give his brother a bad reputation as he did not want him to become a Titan and have his lifespan reduced.
Following the battle of Fort Slava, Galliard watches over Reiner as he recovers from his injuries following the battle. When Reiner awakens, he thanks Galliard for saving him in Fort Slava, but Galliard coldly tells him that he only wanted to keep Marley from losing the Armored Titan. Galliard then explains that he did not inherit his brother's memories, but he did inherit Ymir's memories and harshly berates Reiner for needing to be saved by his comrades so often, including Marcel, whose death Galliard holds Reiner accountable for. Reiner eventually leaves after Pieck joins them, and Galliard stays behind with Pieck.
Mr. and Mrs. Galliard are later seen on the train with the rest of the Eldian survivors.