Lieutenant Koss is a villain in The Rat Patrol. His actor wasn't credited so his name spelling is mostly guesswork.
Koss was a large and brutish German officer serving under Colonel von Bracht. Von Bracht described him as "a psychopath." He delighted in torturing others. He was known to sing while he tortured people, and their pain amused him, causing him to laugh hysterically. Von Bracht didn't like him, but kept him around as he considered him useful at extracting information out of others.
He was with von Bracht when the Colonel was preparing to mount a massive attack against the Allies. The Rat Patrol stumbled across their encampment, and Troy and Hitchcock were taken prisoner, although Moffitt and Tully managed to get away. Not knowing of the existence of the other two men, von Bracht attempted to force Troy and Hitchcock to radio their headquarters and lie to their superiors, saying they didn't find any German troops, to preserve the element of surprise. They refused, and so were turned over to Koss, who prepared to use a blowtorch on them, singing merrily in German all he while.
As he prepared to use the torch on Troy, Hitchcock, in a bid to save his friend, yelled out that he'd make the call. Koss almost went ahead and burned Troy anyway, and had to be stopped by a concerned guard. Disappointed, Koss let up, and the two prisoners were returned to von Bracht's tent. Just as Hitchcock was about to make the call, however, Moffitt, disguised as Italian officer Cristalde, arrived with a "captured" Tully in tow. He and Tully were able to get the drop on the Germans, knocking out Koss and taking von Bracht prisoner.
The Rats absconded in a stolen German truck with the captive von Bracht. As they were escaping, Koss woke up. Drawing his sidearm, he ran out of the tent, shooting at them. Tully returned fire, hitting Koss and killing him.