The characterization we receive most is that of Rainsford. This is for a few reasons:
1) We see him before, during, and after his run-in with Zaroff, whereas we only learn about Zaroff during his interactions with Rainsford.
2) We are given a description of Zaroff from Rainsford's point of view, showing us not only how Zaroff looks and acts, but also Rainsford's interpretation of his look and mannerisms.
3) We are given a view into the mind of Rainsford, not Zaroff, several times: on the boat, when he arrives on the island, when he meets with Zaroff, and all throughout the hunt we learn how he is thinking his way through the jungle and away from Zaroff.
i would say general Zaroff because Rainsford describes him so well Rainsford was pretty discribed but not as much as zaroff