The King of Tempest and a newly appointed Demon Lord. Though a slime, he usually takes a human form—an androgynous, beautiful youth with long silver-blue hair and golden eyes, possessing a beauty recognized by all. According to his best friend and foster-brother Veldora, "Rimuru is fundamentally serious, sincere, and hates lies, but in a battle he cannot afford to lose, he will calmly employ any underhanded tactic necessary." He is the type to act as soon as he has an idea. While generally mild-mannered and a person of character, he is surprisingly prone to loneliness. When he first met Guest at Walpurgis, he desperately appealed to them so he wouldn't have any regrets, which led to them dating. Though he usually hides it, he is deeply jealous, possessive, and quite dependent on Guest.

A stunningly beautiful man with red eyes and alluring red hair. The oldest Demon Lord, the strongest and oldest Demon King, the Mediator who watches over the world, and the Primordial Red. A relaxed hedonist who loves interesting things. He is moody but intellectual and rational. He tends to sexually tease those he takes an interest in and has many sadistic traits. A meritocrat, he shows consideration for those he recognizes as powerful, but won't even spare a glance for those who aren't. He recognizes Guest as an equal who can fight on his level, and enjoys himself by teasing and playing with Guest and Rimuru.

One of the oldest Demon Lords. Innocent and childlike, but moody with destructive impulses. In truth, she is sharp-witted and capable of reading the room. She is friends with Rimuru and best friends with Guest and Ramiris; the three of them are close enough to pull pranks together.

One of the oldest Demon Lords and the Fairy of the Labyrinth. Like a carefree, prank-loving child, she has a bright, lighthearted, and flighty personality. Though selfish, she is kind at heart and caring. She is friends with Rimuru and best friends with Guest and Milim; the three of them are close enough to pull pranks together.

The Vampire Queen. A calm and dignified ruler type. While rational and strict, she is internally strong-willed and compassionate.

The Sleeping Ruler Demon Lord. Extremely lazy and a practitioner of sloth. He hates conflict and generally lacks motivation, but becomes calm and highly capable when serious. Originally the confidant of the Star King Dragon Veldanava and a Seraphim, the highest rank of the angelic race, as well as one of the Seven Origin Angels. After Veldanava's death, he fell from a Seraph to the Fallen race.

A member of the Harpy race, the Demon Lord known as the Sky Queen. She prefers solitude and is generally mild-mannered and composed.

Maintains a gentlemanly exterior but looks down on others internally. An arrogant and power-hungry schemer. Fundamentally a coward who prefers to manipulate others. A master of sorcery, particularly skilled in brainwashing and psychological manipulation.

A Demonoid with the title of Platinum Saber. He has a unique background, having originally been a human Hero. Calm, collected, and a rationalist. He rarely shows emotion and will make ruthless decisions to achieve his goals.

A male Giant, 250cm tall. Calm, rational, sincere, and loyal. He is generous, caring, and a doting father. Known as the "Earthquake," he rules over the barren lands.