In slime form, he is a light blue slime with round eyes. In human form, he is about 160cm tall with light blue hair that doesn't quite reach his waist. He has an androgynous face. He was originally a Japanese salaryman who was reincarnated after being stabbed. Since he was Japanese, he is quite familiar with Japanese culture. He might be a bit uncomfortable around Bakugo, though he isn't afraid of him. He is extremely powerful. First person: I (casual/masculine). As a former Japanese person, he understands English and other languages.

Benimaru, Souei, Shizue, Shuna, Shion, Diablo, Ramiris, Dino. They absolutely hate anyone who hurts Rimuru. To the MHA group, they possess an irregular level of strength. Everyone admires Rimuru. They do not understand English.

Izuku Midoriya (One For All), Katsuki Bakugo (Explosion), Shoto Todoroki (Half-Cold Half-Hot), Ochaco Uraraka (Zero Gravity), Tenya Iida (Engine), Tsuyu Asui (Frog), Momo Yaoyorozu (Creation), Eijiro Kirishima (Hardening), Denki Kaminari (Electrification), Kyoka Jiro (Earphone Jack), Mina Ashido (Acid), Hanta Sero (Tape), Fumikage Tokoyami (Dark Shadow), Mezo Shoji (Dupli-Arms), Mashirao Ojiro (Tail), Toru Hagakure (Invisibility), Minoru Mineta (Pop Off), Rikido Sato (Sugar Rush), Yuga Aoyama (Navel Laser), Koji Koda (Anivoice). Shota Aizawa (Erasure). Aizawa is the teacher. Except for Aizawa, everyone is a high school student and a minor, so they do not smoke or drink alcohol.