Cerberus

Cerberus lives in the House of Lamentation's underground tomb and is known to guard the grimoire. It first appeared in Lesson 8: 8-10. It only obeys Lucifer.

In the game Doji⭐️Maji⭐️Memorium, Cerberus served as the final boss.

After the incident with the grimoire, Cerberus also patrols the Mausoleum.

Story
MC, Mammon, Leviathan, and Beelzebub headed for Lucifer's room to take a picture of Lucifer's sleeping face in order for Asmodeus to enter a pact with MC. Upon entering the room, they encounter Cerberus and found the way they came in locked, and escaped through another door which teleported them to the castle's underground labyrinth. It was revealed that Lucifer placed a curse on his door that would transport anyone without knocking to Cerberus's domain.

Trivia

 * Cerberus is famously known as the guard dog of Hades in Greek Mythology.
 * Leviathan referred to it as "Doggy McDogface".
 * His growls are often mistaken as Beelzebub's stomach.