Diem is a character who makes a one-time appearance in the Season 2 episode titled "The American Dream".
Diem is a Vietnamese immigrant farmer who owns a produce farm in Central Valley, California. Tranh Van Zung, a former Vietnamese Air Force colonel who now owns a crop dusting air service, has been extorting protection money from all the Vietnamese farmers. Without paying the protection money, the farmers won't be allowed to harvest their crops and bring them to market.
However Diem and his friend Tru who own the two largest farms in the valley have had enough of Zung behaving like the warlord he used to be and expecting tribute from his subjects. He believes that if he and Tru stand up to Zung and refuse to pay up, Zung will be finished. So he harvests his crop and drives his produce truck to deliver it to market accompanied by his son Tommy. Zung, obviously cannot let this challenge go unanswered. He sends one of his thugs, Ko to deal with it. Ko flies over Diem's produce truck in his crop-duster, spraying it with gasoline and then throws a incendiary bomb, blowing it up. Both Diem and Tommy are killed.