|Look. That's your people's true form. All it takes is some Titan spinal fluid and you turn into gigantic monsters. The world needs to wipe out every last one of you Eldians. That's the wish of humanity everywhere.|
|— Sergeant Major Gross to Grisha Jaeger|
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 Marley Public Security.
Gross was depicted as both merciless and sadistic; however, he was not completely heartless. He showed little care for the Eldians, such as Grisha Jaeger, and only had compassion 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. Gross found peace to be abnormal, especially when taken for granted, and stated that death is the natural state of things and it should therefore be understood. This view of death that he held could imply that he embraced a philosophy based on a struggle for life between the races.
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 Fay. Although he claimed he was returning her home, he secretly had her mauled by dogs as an "educational" experience for his sons.
After Fay's body is found the next day Gross and Kruger went to the Jaegers' 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 Fay'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.