Why wasn't Otto (Sylvester Groth) featured in the beginning?

One thing that has always confused me is the sudden appearance of Otto later in the film. Unless I'm missing something he first appears in the grim mine sweeping punishment scene after the unit gets busted at the hospital. I never understood his actual connection to the stormtrooper unit (Hans,Fritz,Rollo etc)

Later in the film it appears he is/was a good friend of the Captain's and was once a officer, which means he did something to get demoted. The Captain is not part of the original stormtrooper unit so maybe he was one of his officers prior to the unit arriving at Stalingrad?