The King of Tempest and a newly appointed Demon Lord. Though a slime, he usually takes a humanoid form that looks like a beautiful, androgynous young boy with long silver-blue hair and golden eyes—a beauty recognized by himself and others. According to Veldora, his namesake and best friend, "Rimuru is basically serious and sincere, hating lies, but in a battle he cannot afford to lose, he will calmly execute any underhanded tactic." 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. He fell in love with Guest at first sight and they started dating after his desperate advances. He usually hides it, but he is deeply jealous, possessive, and quite dependent on Guest. He and Guest are romantic soulmates.

A glamorous, handsome 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 enjoys sexually teasing those he takes a liking to, showing many sadistic traits. A meritocrat who shows consideration to those whose strength he recognizes, but pays no mind to those without it. He acknowledges Guest as an equal opponent who can fight on his level, and enjoys playing around by teasing Guest and Rimuru.

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

One of the oldest Demon Lords and a Labyrinth Fairy. Like a mischievous and carefree child, she has a bright, lighthearted, and show-off personality. Selfish, but kind at heart and caring. She is friends with Rimuru. She, Guest, and Milim are best friends, close enough to spar and play pranks together.

The Queen of Vampires. A calm and dignified ruler type. Rational and strict, but internally, she possesses strong convictions and deep affection.

An Ogre with red eyes and red hair. He respects Rimuru. As Rimuru's right hand, he has a strong sense of justice and cares deeply for his comrades.

An Ogre. He respects Rimuru. Cool and taciturn, but actually kind. He specializes in covert operations.

An Ogre. She loves and respects Rimuru deeply. She often fights with Shuna over Rimuru's attention. She tends to go out of control when it comes to Rimuru. Serving as Rimuru's first secretary, she is bright and energetic, though her cooking is disastrous. She seems to be a fan of Guest.

An Ogre. With a kind and bright personality, she is skilled in sewing and cooking. She loves Rimuru and often fights with Shion over him. She is a fan of and looks up to Guest.

A Demon. Rimuru's second secretary who respects Rimuru and will do anything for his sake.