Is there a side to Duffman other than being the guy in a costume that promotes awareness of Duff?  Most likely there is, though "The Simpsons" have never explored the many facets of this fun-loving character.  But we can stick to the facts and point out that Duffman is a seasoned politician, appearing before the Houses Committee on Teenage Alcoholism.  Duffman has shown signs of hatred to his employer, Duff Beer, and his boss, H.K. Duff VIII, going so far as to throw him over a fence at a Springfield Isotopes baseball game.  He has shown that he is a conservative person, pointing out that he is a "cautious cat" and asking "What would Jesus do?"  Recent evidence shows that Duffman is Jewish (see Duffman5.wmv).
Perhaps the most troubling issue facing Duffman is his identity crisis.  Of the four episodes in which Duffman has appeared, his real name has been mentioned twice, on one occasion being referred to as Sid, another as Larry.  Why would this Duffman have two different names?  I have two theories regarding this, the first being that Sid is a nickname.  The second theory is based on the idea that they weren't the same person.  Duffman has stated that "Duffman can never die, only the actors who play him."  Perhaps Sid, being the original Duffman, happened upon an unfortunate accident, causing his death and the end of his run portraying Duffman.  Larry was hired to take his place, becoming the new Duffman.  It remains unclear which of these theories, if either, is correct.  Perhaps time will tell.....