These guys make up the Actionist Party on Foundation. They don't like the way Hardin is preparing the Foundation for possible conflict (which is, not at all), so they propose that they will instead take action and prepare for an all-out, action-packed war. So, they're pretty obviously antagonists to not only Hardin but also the novel's theme of nonviolence. Now if only we could figure out why they call themselves the Actionists….
Of all the guys listed above, you only really need to remember two: Lewis Bort and Sef Sermak.
Bort worked as an undercover agent on Anacreon. His report to the Actionist Party in Chapter 4 clues us into the Foundation's spiritual power as well as its place in Anacreon society.
Sermak acts as the mouthpiece for the Actionist Party. He's their leader, so he's the one who goes toe-to-toe with Hardin in verbal boxing matches. Unlike Hardin, he wants to weapon up and attack Anacreon, so he's a lot more like Wienis than other Foundation leaders.
King Lepold I is the young king of Anacreon and Wienis's nephew. In Anacreon society, he is second only to the Galactic Spirit, and his kingship supposedly results from divine right.
The King may be king and all, but he's not a very good one. First, he spends more of his time hunting and playing games than actually, you know, ruling the Kingdom. As a result, his uncle Wienis runs the state behind his back. Since Lepold finds his uncle a tad intimidating, Wienis basically does what he wants.
After watching his uncle blast himself into smithereens, Lepold signs a treaty with Hardin, perhaps hoping he'll keep his upper torso in the deal.
High Priest of Anacreon and ambassador to the Foundation, Verisof is a friend of Hardin and is instrumental to the mayor's plan to defeat Anacreon. He may be a High Priest, but Verisof is more into science than mysticism. He doesn't buy into King Lepold's divine right, so starting a rebellion was probably pretty easy for him.
Aporat is the head priest aboard the Wienis space cruiser. Aporat believes attacking the Foundation to be sacrilege, even going so far as to curse the ship. Then he does what anybody would do and leads a mutiny against Prince Lefkin, taking over the ship and preventing all-out war. No biggie.
Aporat is the only character we see who actually believes in Hardin's religion. His chapter may be short, but it lets us access the mindset of those devout to the Foundation's religion. When we see how adamantly Aporat views his religion, we get why Hardin's plan worked out.
The leader aboard the space cruiser Wienis, Prince Lefkin tries to rally the troops back to his side during Aporat's uprising. But he stands up to Aporat's religious zeal. Things head south for Lefkin quickly, as he's beaten up and forced to record a message of surrender for the Anacreon navy.