Chen is the lead judge at Seldon's trial, the Chief Commissioner of Public Safety, and just maybe the one true power of the Empire, who uses the child emperor merely as a symbol (1.7.7).
We only really get to know two things about this guy: first, he's a politician and second, he doesn't like what Seldon says about the Galactic Empire being destroyed. So, it's hard to peg him as a villain, although he is surely an antagonist.
He's against what Seldon is saying, but it has more to do with his job than anything else. After all, it's kind of hard to get reelected when anarchy and chaos are inevitable. So, while he doesn't want Seldon prophesying gloom and doom, he also helps Seldon complete his master plan by sending Seldon and his people to Terminus.
See what we mean? Antagonist, sure, but not a bad guy per se. Remember, without an antagonist, there's no story.
Lors is Gaal's lawyer and as such he does cliché lawyer-type things. When Gaal is being interrogated, Lors comes in with a voice recorder and a "have you talked to anyone?" attitude. During Seldon's trial, he's technically Seldon's lawyer, but he doesn't say anything. If you've ever seen a lawyer drama on TV, then you've got this guy figured out.
Jerril works as an agent for the Commission of Public Safety. He spies on Gaal during his trip to the observatory and confirms that Gaal is working with Seldon. Afterward, he disappears from the story like a rabbit into a magician's hat. Poof!