|Get a little Titan spinal fluid in your blood and you become giant monsters. And you think you're just as human as us? There is no living thing in the world like you Eldians, you "Subjects of Ymir." You're monsters in human skin. And it's a terrifying nightmare that anyone ever let you reproduce.|
|— Sergeant Major Gross to Grisha Yeager|
Gross was a short and fairly chubby middle aged man with a balding head and short black hair on the sides. He had light-colored eyes, a mustache, and mild wrinkles on his forehead and around his eyes. On duty, he wore the standard uniform for Marleyan soldiers.
Gross was depicted as somewhat sadistic, however, he was not heartless. He showed little care for the Eldians such as Grisha, and only had some sympathy for his family. His goal was to completely eradicate the world of Eldians. Gross was very impulsive in his way of speaking. He was extremely racist and prejudiced toward Eldians and often justified his inhumane actions by using history of oppression of Eldians toward the Marley to justify the need for revenge of the Marley to the Eldians. These actions extended to the fact he willed to kill an innocent girl just because she was an Eldian. In the end, for him, the Eldians were the enemy of mankind and they needed to be eradicated.
When Grisha Yeager and his little sister Faye wandered outside of the Liberio internment zone without an exit permit to see a blimp, Gross and his fellow soldier, Kruger, were slacking off by the riverside and they spotted and caught them. Kruger delivered physical punishment to Grisha while Gross walked away with Faye.
However, her mutilated body was found the next day in the river and Gross, accompanied with Kruger went to the Yeagers' house and informed them of the death of their daughter. Gross claimed that he took Faye to the border of Liberio and that it was her own fault for wandering around town without permission, to begin with.
Fifteen years later, after Grisha's son had informed on his parents and the Eldia Restorationists to the Public Security Authorities, they were all caught and received life sentences for the crime of treason and were brought by a steamboat to the borderline wall of Utopia to be turned into mindless Titans. Gross, atop the wall with Kruger and other soldiers, was annoyed with a Restorationist's yelling and kicked him off the wall while the other soldiers started injecting the rest of the Restorationists with Titan serum and watched as they all turned into Titans and started chasing after him while Grisha yelled at them to stop and recognize Grice. Gross told Kruger to turn Grisha into a Titan but Kruger responded that he still wanted to interrogate him.
Grisha's wife was brought to be injected and Gross pitied that it was a woman and said that if only she did not have Devil's blood in her and when Grisha saw his wife, he started yelling that he told the Marleyan soldiers everything about her being of royal blood and that she ought to be valuable for Marley, but Kruger quickly slammed his head on the wall's surface and silenced him while Gross said he could not stand his nonsense anymore and told Kruger again to shut him up and turn him into a Titan. Gross proceeded with injecting Dina and pushed her off the wall while Grisha watched his wife turn into a Titan. Grisha then confronted Gross about his little sister and said that he was the one who killed her and fed her to dogs.
Gross ordered all the other soldiers to get back to the boat and he said he remembered Grisha and said that he would not turn him into a Titan and rather have him get eaten by the last Restorationist after turning him into a 3 or 4 meter Titan so Grisha could fight him. Grisha asked Gross why was he doing this and he said because it was fun to watch humans get eaten by monsters and that he needed that entertainment in his life. Gross injected the last Restorationist and told Grisha that it was education when he fed his little sister to his sons' dogs and thanks to that, they grew up to be fine young men.
Gross told Grisha to look down at the Titan and claimed that that is what the Eldians really were and all that it took was some Titan spinal fluid and they turn into giant monsters, with Eldians being the only creatures like that in the world and that humanity wished for every last Eldian to be eliminated from the world. Grisha said that it was a lie and that Marley was distorting history but Gross dragged him to the edge of the wall and as he was about to throw him, Kruger, to Gross and Grisha's shock, pushed Gross off the wall instead and he rolled down the sand dunes and the Titan he just turned for Grisha, grabbed him and started eating at his face.
- "Sergeant Major" is the highest non-commissioned officer rank in the real world's military forces. Gross' holding of this rank may suggest that the military rank structure in Marley is similar to that of a military in the real world. It also implies that brutality against the Eldians in Marley is not discouraged in the Public Security Authorities, as Gross was able to keep, or able to progress into, that high rank after the murder of Faye Yeager with little to no repercussions.