As the head of Phillips Broadcasting, Duke shows many traits typically associated with business villains such as a willingness to retool anything (often Jay's show) for profit, being corrupt, and acting like an evil overlord. He is shown to have incredible strength and physique, such as walking right through two walls and throwing Jay like a medicine ball during an exercise routine. He also bullies his employees, notably Jay whom he frequently antagonizes for allegedly being gay and into him (Jay isn't and Duke later reveals that he does it just to bully him) which didn't stop even when Jay started dating his second season girlfriend Alice Tompkins.
Although Duke has shown a human side as well. He once built a preschool to accommodate Alice's daughter after she had been rejected from several prestigious schools (noted as having been "built on a dare"). Duke has also shown moments of appreciation for Jay, such as the time when Jay developed a cure for Duke's life-threatening disease in an episode parodying Lorenzo's Oil. He also constructed a hospital/medical research center, although it was accompanied by a large statue of an Atlas-esque Duke repeating the phrase "All Hail Duke! Duke is Life!" In one episode, he was also shown as being a bit ill-informed about the average person, believing the Average Joe made three hundred grand a year.
That said, he has hinted that he is not above criminal activities for the sake of profits. Once during a trial, in a parody of A Few Good Men, Duke accidentally revealed that he had sold mustard gas to Muammar Gaddafi (then retracting it as being that "mustard gives him gas"). In order to get the late Saddam Hussein to appear in a multi-star action film, he gave the dictator several bombs. When Jay's mother Eleanor published a book called The Fat Little Pig, which featured an unflattering caricature of Jay, a crowd of the book's fans later tried to attack Jay twice (once for resembling the pig, the second time for making them feel sympathetic for Jay). Duke did nothing to help his employee, instead only telling the crowd to "Make him squeal!" He also once revealed to Jay that he owns a baseball team he bets against and is registered as a citizen of the Dutch Antilles to avoid paying taxes. He can also be quite petty and cheating, such as winning the America Cup with a motor on his boat and paying the Webster Dictionary people to make his word "Quzybuk" legitimate so he could get a high score in Scrabble.In "All the Duke's Men", Duke is inspired to run for President and has a brief daydream where he's encouraged by a devilish version of himself to run and "drop a mess of bombs" while in his office. His methods of gaining support during the campaign are underhanded and crude, such as using his "evil eye" to silence a reporter questioning his phony marriage to TV mom June Lockhart (who then referred to Duke as "evil one" and asking how he may serve him) and having campaign posters featuring unmotivated Irish bashing (him kicking a leprechaun while proclaiming that the Irish "suck"). Towards the end of the episode, he finally declares his true intentions over national television.
"First, I’ll tell you what I’m really gonna do as President. I’ll run this country like I run my company. I’m gonna raid the pension fund, dump chemicals in the ocean, and sell our best assets to the Japanese! Half of you states are in the toilet, and you’re not coming out! New York, you know what I’m talking about! California, kiss your smoggy butt goodbye! New England, you’re going back to Old England!"
As expected, this causes him to lose his Presidential nomination and ends his phony marriage to Lockhart who then proceeds to sic a vicious Lassie on him.
In the penultimate episode "Dukerella", Duke falls in love with Alice's sister Miranda. The episode does have a running gag referencing the time he kept Jay locked up in a dungeon for wanting a dental plan (long enough for Jay to have grown a thick beard). When approached once in the episode, he projected his face in a Wizard of Oz parody and menacingly asked if Jay was going to ask about the dental plan once again. At the end of the episode, Duke tells Jay that he'll grant any request in exchange for getting him and Miranda together. Jay requests that dental plan. Predictably, Duke rejects it and has Jay locked in the dungeon once more.
Duke shows one final villainous (and rather cowardly) act in the finale "I Can't Believe It's a Clip Show" where Jay's tribute show is cut short by a band of terrorists. Duke manages to escape after offering to get ransom money but leaves Jay and the others to fend for themselves to save his own skin.