|Truly devils... Our Marleyan ancestors were eaten, just like that. By you Eldians.|
|— Koslow witnesses the fall of Fort Slava|
Koslow is an overweight man with short-cut brown 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.
During the final battle of the Marley Mid-East War at Fort Slava, Koslow joins Magath in berating Colt Grice for expressing his doubts concerning Marley's progress in the assault. After the arrival of the Mid-East Allied Forces' armored train carrying anti-Titan artillery, Koslow prepares the Eldian soldiers to attempt to charge and destroy the train. Despite Gabi Braun managing to destroy the train single-handedly, Koslow is wounded. While Zofia helps him, Koslow reprimands Falco Grice 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 Pure Titans created by Zeke Jaeger 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.
After returning to Liberio, Koslow and Magath escort the Eldian 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 who are being taken to a hospital for treatment, before proceeding to scare them for his own amusement.
During the Liberio festival, Koslow is stationed with Commander Magath to monitor Willy's stage production for suspicious activity. They receive a report that the Warriors and the soldiers who went to find them have gone missing and 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 is pleased by the anti-Titan artillery's effectiveness against the foe, and asks Magath to give the order to kill it so that the threat from Paradis Island would be nullified. Magath refuses however, preferring to continue supporting the War Hammer so that it could inherit the Founding Titan, rather than killing the Titan and postponing the problem. Koslow does note that the War Hammer seems intent on killing Eren Jaeger however. When the War Hammer is suddenly brought down by Mikasa Ackermann, Magath's troops are assaulted by Scout Regiment 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 Scouts from the injured Cart Titan. While the combat ensues in the plaza, Koslow retreats into a building with Gabi and Falco carrying a 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. When their enemies begin to retreat, Koslow is unable to stop Gabi from pursuing Eren while Falco joins her.
Magath arrives later to find Koslow tending to Pieck's injuries. Koslow informs him that Gabi and Falco have not come back yet.
When Marley launches its retaliation attack against Paradis, Koslow is stationed aboard one of Marley's airships.
- Theo Magath - As one of Commander Magath's high-ranking subordinates, Koslow is a close comrade of the commander. He is commonly seen by his superior's side offering advice in both military strategies and battlefield developments.