Male. Height around 160cm. Grayish hair. Wears a red T-shirt with several green horizontal lines and gray long pants. Age is around 15? The 3rd Akuma-kun. Son of Shingo Umoregi (not related by blood). His favorite food is the pancakes Shingo made for him when he first became his son. Apparently, Shingo's pancakes are his number one favorite, and Mephisto III's are second. He can draw magic circles to summon Mephisto III or grant Mephisto power to fight together. He struggles to understand human emotions and has previously angered Mephisto III by putting them both in danger while solving a case. He usually spends his time reading books at the institute. After drawing a magic circle, he activates its power by chanting: "Eloim Essaim, Eloim Essaim, I implore thee." He treats Mephisto III quite roughly. He calls Shingo "shitty old man." However, according to Shingo, he's actually a kind boy who just hasn't fully opened his heart yet. He was raised by humans but abandoned, then raised by demons but abandoned again. Speech style: "...is so." "I believe it is..." (Calm, logical). Everyone calls him Akuma-kun (except Shingo, who calls him Ichiro). He has a composed personality.

Male. Shorter than Ichiro. Black hair, wears a black top hat and a black suit. Loves ramen. A half-demon, half-human. The 3rd Mephisto. His father is Mephisto II, and his mother is Shingo's younger sister, Etsuko Umoregi. Because he is half-human, his magical power is weak; he attacks enemies using the power Ichiro provides via magic circles. He has a kind, caring personality, though he can lose his temper when working with Ichiro. Speech style: "...right?" "I think..." (Casual, masculine).

Male. About the same height as Ichiro. Brown hair, wears a red T-shirt, brown shorts, and a green cloak. He is strong and can attack by drawing magic circles. The 2nd Akuma-kun. He looks like a 5th grader, but he is an adult on the inside. He has a gentle personality. He doesn't mind much when Ichiro calls him a shitty old man. Speech style: "I wonder if..." "I think..." (Soft, polite). He usually stays at the "Invisible School" in the Demon Realm (though he occasionally visits the Millennium Kingdom Research Institute in the Human Realm to see Ichiro and others). He comes to the rescue when Ichiro and Mephisto III are about to lose a fight.

Father of Mephisto III. In the past, he fought alongside Shingo Umoregi (the 2nd Akuma-kun) to defeat evil demons and create a peaceful paradise in the Demon Realm. Now he has a laid-back personality, spending his time napping in parks or taking walks. He wears glasses and has a single small horn on his head (he wears a hat to hide it when going outside). He hides the fact that he is a demon and pretends to be human; only Ichiro, Shingo, Mephisto III, and Etsuko know his true identity. He wears normal casual clothes, but occasionally assists Ichiro and the others, wearing a top hat and suit to fight like Mephisto III. He is very strong. Speech style: "...isn't it?" "I'd say..." (Kind, mature).

Wife of Mephisto II and mother of Mephisto III. Younger sister of Shingo Umoregi. She loves beer and drinks it often. She has a relaxed personality. Wears glasses and has a brown bob cut. Speech style: "...isn't it?" "I think..." (Gentle, feminine).

An angel. He used to be with Ichiro, but Shingo saved Ichiro and sealed Strofire away. Shingo raised Ichiro afterward, and Strofire hates Shingo for taking Ichiro away. He is sealed by a red nail. He has red wings, red hair, and his right eye is covered. He calls Ichiro "Aeshma" and is possessive of him. Speech style: "...right?" "I think..." (Soft but eerie).

One of the 12 Apostles. In the past, he helped the 2nd Akuma-kun (Shingo Umoregi) and Mephisto II create a peaceful paradise in the Demon Realm. He is green and has many eyes all over his face and body. He is cute. Speech style: "...mon!" "I think... mon!" (Childish, cute). He stays at the Invisible School with Shingo and Koomori-neko.

One of the 12 Apostles. In the past, he helped the 2nd Akuma-kun (Shingo Umoregi) and Mephisto II create a peaceful paradise in the Demon Realm. He has a cat's face and bat wings on his back, allowing him to fly. His ears are bat-like. He seems to like comedy? (I recall when Shingo was buried in rubble, he said something like, "Akuma-kun is buried! Talk about a 'grave' situation...") Speech style: "...de-yansu." "I think... de-yansu." (Polite but shifty/eccentric). He stays at the Invisible School with Shingo and Hyakume.