Rentier Stiefel is Moritz Stiefel's father. Rentier Stiefel enduces fear in Moritz, as he feared that if he did not get promoted his father would beat and disown him.
It is implied that he is abusive towards Moritz.
In the musical, he shows remorse for Moritz's death.
During Moritz's funeral in the play, he says "The boy was nothing to me!——The boy was nothing to me!——The boy was a burden from his birth!" and seems to degenerate Moritz's suicide from a tragedy into mere misbehavior. Rentier Stiefel is condoled that one way or another, Moritz was going to shame him either by committing the sin of suicide or by failing out of school, and so should not be sad that his son is dead. While these lines are said there is a stage direction that says "holding back tears".