Prosser is a human version of Prostetnic Vogon Jeltz, just with a more boring name. Like the Vogons, he's in charge of tearing something down; and like the Vogons, he's very invested in the notion of doing things according to the correct, bureaucratic rules.
Let's look at an example of that love of bureaucracy. Arthur argues that he wasn't properly notified about the impending destruction of his house, and the humans argue that they weren't properly notified about the impending destruction of Earth. Prosser and the Vogons both answer these complaints in the same way, and we can prove it by giving you this test: who says "the plans have been available in the local planning office" (1.31), and who says "All the planning charts and demolition orders have been on display in your local planning department" (3.75)? It's hard to tell since they use very similar language. (Answer: Prosser; the Vogons.)
But while the Vogons are powerful and cruel, Prosser is more easily defeated. Although he's descended from Genghis Khan, the great conqueror (see our Shout-Outs section), "the only vestiges left in Mr. L. Prosser of his mighty ancestry were a pronounced stoutness about the tum and a predilection for little fur hats" (1.17).
This nicely shows us what a ridiculous and comic figure Prosser is: rather than conquering Asia with his Mongols, Prosser is reduced to… wearing a hat. (Unfortunately, a hat doesn't much help against a Vogon Construction Fleet.) Although he is a bully in some respects, Prosser's not as bad as old Genghis was.