|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 of the Public Security Authorities.
Gross was depicted as a sadistic and cruel man. He showed little care for the Eldians such as Grisha, and only had some sympathy for his own family. His goal was to completely rid the world of Eldians. Gross was very impulsive in his way of speaking. He was extremely racist and prejudiced against Eldians and often justified his inhumane actions with the oppression of Marley at the hand of the Eldian Empire to frame his actions as revenge for this oppression. He had no qualms at all to kill an innocent girl just because she was an Eldian. In the end, for him, Eldians were the enemy of mankind and needed to be eradicated as such. Gross found peace to be abnormal, especially when taken for granted, and stated that struggle and death are the natural state of humanity and therefore shouldn't be swept under the rug. This view of struggle and death that he held could imply that he embraced a philosophy based on a struggle for life between the races i.e. racial social darwinism.
While Gross and Kruger were slacking off they noticed two Eldian children outside of Liberio's internment zone without permission. Kruger delivered physical punishment to Grisha while Gross walked away with Faye. Although he claimed he was returning her home, he secretly had her mauled by dogs as an "educational" experience for his sons.
After Faye's body is found the next day Gross and Kruger went to the Yeagers' house and informed them of the death of their daughter. Gross claimed that he took Fay to the gates of Liberio and that it was her own fault for wandering around town without permission.
Fifteen years later the Marley Public Security was tasked with bringing a rebellious group of Eldians to Paradis Island to turn them into Titans. Gross grew 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. Gross told Kruger to turn Grisha into a Titan but Kruger responded that he still wanted to interrogate him.
After injecting a female Restorationist Gross was confronted by Grisha about his hand in Faye's death. Remembering Grisha, Gross decided not to turn him into a Titan and instead force him to fight one of his transformed comrades. 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, proclaiming that the world wanted Eldians eradicated because of the danger their ability to turn into Titans posed. Gross began to throw Grisha from the wall, but was himself pushed off by Kruger. Landing in front of the Titan, he had just transformed, Gross is immediately devoured.