|They truly are...devils. Our Marleyan ancestors were eaten up that way, too. By you Eldians.|
|— Koslow witnesses the fall of Fort Slava|
Koslow is a man of average height with short-cut dark hair, a round face, and a bulbous nose. When on the battlefield, he wears the typical uniform and helmet of the average Marleyan officer.
Koslow is an observant and staunch soldier of the Marley military. In times of war, Koslow is often seen at the side of Commander Magath, his superior officer to whom he provides information and advice. His more notable trait, however, is his bitter demeanor towards the Eldians, whom he considers to be devils, even his subordinates in the Warrior unit. While still showing some degree of respect towards the Warriors and an understanding of their reckless choices as youths, Koslow frequently denounces and belittles his Eldian comrades for their ethnicity, even displaying overt cruelty towards them at times.
During the final battle of the Marley Mid-East War at Fort Slava, Koslow participates in the combat alongside his superior officer, Commander Magath, berating candidate Colt for expressing his doubts concerning Marley's progress in the assault. Towards the end of the battle, Koslow announces the sighted arrival of the Mid-East Allied Forces' armored train carrying anti-Titan artillery. Not long afterward, when the armored train is destroyed and the Warriors press the advantage, Koslow reprimands candidate Falco for attempting to save a wounded Allied soldier. When the soldier denounces his Eldian saviors in his native tongue, Koslow openly laughs at his subordinates' expense. Later, when a Marleyan airship drops Titans created by Zeke Yeager onto the fort, Koslow reminds the Warrior candidates present that their Eldian ancestors have been the cause of the same sort of horrors they are witnessing before their eyes.
During the train ride back to Marley, Koslow grows irritated at the loud celebrations coming from the Warrior unit and suggests to Magath that they should shut the Eldians up, though Magath seems content with allowing them to continue for the time being. When the train arrives in Liberio, Koslow and Magath escort the Warrior unit to the internment zone in the city. After their arrival, Falco asks Koslow about a line of injured soldiers being led into the internment zone. Koslow informs him that the soldiers are injured Eldians suffering from post-traumatic stress that are being taken to a hospital for treatment. He goes on to explain that the time spent digging trenches and coming into close proximity with machine gun fire and bombardments has led to severe mental trauma, a sensitivity which he cruelly exploits for his own amusement.
One month later, during the Liberio festival, Koslow is stationed with Commander Magath to monitor Willy Tybur's stage production for suspicious activity. Eventually, soldiers report that the Warriors and the soldiers who went to find them have gone missing. Magath orders Koslow to mobilize the reserves for a search. Not long afterward, the Attack Titan bursts forth from the stage and begins a massacre. When soldiers rush to Commander Magath to report the details of the situation, Koslow remains at Magath's side as the latter formulates their counterattack.
As the War Hammer Titan battles the Attack Titan, Koslow becomes worried about Magath's use of anti-Titan artillery against the Attacker, concerned that they will lose the Founder in the process. Magath states that Marley's new military policy will not rely on the power of the Titans, instead choosing to rid their enemies of the Founding Titan first and foremost. Koslow is awestruck when the War Hammer easily defeats the Attacker with a single blow to the head, anticipating the Attack Titan's imminent death. However, the War Hammer is suddenly brought down by Mikasa Ackerman, who destroys its nape with a barrage of Thunder Spears. Before Magath's troops can react, they are assaulted by Survey Corps soldiers who begin a firefight on the rooftop.
Sometime later, after having been separated from Commander Magath, Koslow returns to the Liberio plaza to fend off the Survey Corps from the injured Cart Titan. While the combat ensues in the plaza, Koslow retreats into a building with Gabi and Falco carrying the wounded Pieck. Koslow warns Gabi to get away from the window lest their enemies discover their position, though she and Falco disregard him and watch the battle below. Koslow watches in horror with the children as Eren Yeager forces the Jaw Titan to break open the War Hammer Titan's crystal, consuming its keeper. The children cry out for Reiner when Eren prepares to eat the Galliard to steal the Jaw Titan, and Koslow is unable to silence them. They later see an approaching airship gathering their enemies for a retreat, to Koslow's surprise. He is unable to stop Gabi from pursuing Eren while Falco joins her. When Magath returns, Koslow is seen helping Pieck with her injuries and informs him that Gabi and Falco have not come back yet.
- Theo Magath - As one of Commander Magath's high-ranking subordinate, Koslow is a close comrade of the commander. He is nearly always seen by his superior's side offering advice in both military strategies and battlefield developments.