If you peg Dr. Heidegger as the protagonist, then you probably think of the guests as the antagonists. These are not your typical villains though – it's more accurate to describe the four individuals as foolish rather than antagonistic. They don't threaten Dr. Heidegger in any way; they just provide a negative example in contrast to his positive one.
A very different interpretation of "Dr. Heidegger's Experiment" reads Dr. Heidegger as a sort of villain. He cruelly manipulates his four guests as though they were rats in a lab. He toys with their desire to be young again, pretends to give them what they want, and feels no regret when they are left with nothing. The sinister objects in his office – the skeleton in the closet, the mysterious black folio – along with the suspicious death of his bride and the apparently abundant number of dead patients adds to the shadowy nature of his character.