That Time I Got Reincarnated as a Slime tells the story of Rimuru Tempest, an isekai hero with incredible powers and the political skills to run a fantasy nation of his own. Rimuru has become a highly consequential figure, allying with many nations while earning the ire of others. The council of Demon Lords has taken notice of Rimuru as well.
The Demon Lords are among the world's most powerful beings, and they often rule their own kingdoms or even religious orders of all sorts. Rimuru Tempest plans to join them as the rookie Demon Lord, and he has a mixed relationship with this bunch by the time the Walpurgis meeting begins. Some are his enemies, others are allies, and a few are still undecided or neutral toward him.
Demon Lord Allies Of Rimuru: Milim, Ramiris & Carrion
Rimuru Tempest met Demon Lord Milim Nava in the first season, winning her over as a friend with the promise of snacks and fun, which appealed to Milim's childish personality. Milim has since been a loose ally of the Tempest nation and has made other friends there, but this is not a permanent or official alliance, and Rimuru and the others must work hard to keep Milim on their side. Recently, Milim fell under the spell of Clayman, becoming his puppet due to a mind-control necklace.
Demon Lord Carrion, who rules the animal kingdom of Eurazania, is another ally of Rimuru's, being part of Rimuru's international coalition. He met Rimuru through the Beastketeers, who served as ambassadors to Tempest, but Carrion is currently in dire straits as a captive in Clayman's castle.
Demon Lord Ramiris is another rally of Rimuru's, a fairy-like being that watched as Rimuru harnessed the magic of Shizu's young students. Ramiris won't help defend Tempest, but she did warn Rimuru of Walpurgis, helping Rimuru to form his plans to storm the council and confront Clayman personally.
Demon Lord Enemies Of Rimuru: Clayman, Frey & Leon Cromwell
Demon Lord Clayman is Rimuru's greatest enemy as of Season 2, and he was responsible for the bloody Falmuth/Church assault on Tempest. Clayman wants Rimuru and his kingdom dead, and he aspires to become a True Demon Lord by invading Eurazania and harvesting thousands of innocent souls to fuel his ascension. Rimuru teleported his army to that nation to block Clayman's invading forces. It was Clayman who called for the most recent Walpurgis.
Demon Lord Frey is the harpy queen, a powerful winged humanoid who helped the possessed Milim raid Eurazania and defeat Demon Lord Carrion in battle. She is allied with Clayman but appears unenthusiastic about this alliance, and she predicts that Rimuru and Milim will take Clayman down.
Demon Leon Cromwell is an ex-human member of the council who summoned Shizu from World War II-era Japan and placed the Ifrit fire spirit inside her, which hastened her death. Rimuru is determined to avenge Shizu for this crime, and Cromwell is troubled that Rimuru is actively blocking most nations from summoning more Otherworlders from Earth. This places him and Rimuru on a collision course.
Neutral Or Unclear Demon Lords: Dino, Dagruel, Guy Crimson, Karazeam & Roy Valentine
Five more Demon Lords exist for a total of 11, though not all are necessarily part of the council; when Rimuru reviewed all the Demon Lords with Veldora, he counted only 10. The one he didn't mention was Demon Lord Kazaream, who inhabits a female elf body and is allied with Yuuki, who has sold Rimuru out. It's possible that Kazaream is an enemy of Rimuru's, but nothing is definitive yet.
The remaining Demon Lords have barely appeared in the Slime anime thus far, and it's not yet clear where they stand with Rimuru Tempest as Wapurgis draws near. One of the oldest and strongest, Guy Crimson, is somewhat sympathetic to Cromwell, an enemy of Rimuru's, but it's unclear if Guy will take any action against Rimuru on Cromwell's behalf. Given his position and personality, it seems unlikely. Then there's Demon Lord Roy Valentine, who heads the Ruberios Empire, the home of Hinata Sakaguchi the knight. That suggests hostility toward Rimuru on Valentine's part, but again, nothing is yet confirmed.
Finally, Demon Lords Dino and Daguel are complete unknowns, since they were introduced only in the most recent episodes. They may simply be bystanders as Rimuru clashes with Clayman and Cromwell, and the same may be true of Guy Crimson. Alternatively, they may take advantage of the power vacuum left by someone's death to become rivals to Rimuru -- a possibility that Rimuru must account for as Walpurgis begins.