Every story has its bad guys. They’re often a one dimensional, single minded villain that wants something like money, or power, or world destruction. An antagonist who everyone hates and whose defeat drives the plot. In the case of the Diablo games, it’s Diablo himself.
And while Big Red is nasty in appearance and intent, he’s not much for conversation. In fact, during the series so far he’s had no more then three lines of dialogue! Some might not even count the speech of the Dark Wanderer as Diablo, in that case, two!
We love our bad guys, but occasionally we like them to be a little less… “Grrr I’m bad, kill me”.
That’s where the Douchebags come in (Note: not an official authoring term). Stories often need side characters to give one-dimensional-bad-guy some support. Sometimes they’re working for him as ultra-skilled or ultra-humorous body guards. Sometimes they’re protagonists that can never get things right. More often then not they’re double-undercover-secret-triple-serial-side swappers, who started off good but turned bad on the road to revenge (and will probably come good again in the end).