Who is the man who first opens the door? What is the difference about Ivan, other than his size and beard?

The man who opens the door is Zaroff's servant, Ivan.

The first thing Rainsford's eyes discerned was the largest man Rainsford had ever seen--a gigantic creature, solidly made and black bearded to the waist. In his hand the man held a long-barreled revolver, and he was pointing it straight at Rainsford's heart.