His job is to make sure the Rabbids' prisoners don't escape.
Serguei's first (and what seems to be only appearance) is in Rayman Raving Rabbids. In the Wii he is the main enemy and Rayman's jailer. He often calls Rayman out for challenges and such other various party challenges. Despite his huge body and scary appearance he is very harmless, though this may be because he is not allowed to hurt Rayman. In the DS version Rayman's main plan is to find the "Rabble Droid" and defeat him.
Serguei does not appear in the second game as the main enemy and is instead replaced by a General-like Bunny with one red eye and one natural eye. He is, however, on the American box art climbing a tower referencing King Kong.