Cerberus

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

Cerberus is a family pet of sorts for it’s seen in chat messages that Lucifer often takes him for a walk and cares for him, like a regular dog. However, Cerberus is implied to be very huge, for Leviathan commented that he felt hot air coming above him.

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 into Cerberus's domain.

When MC, Mammon, Leviathan, Satan, and Lucifer enter the game Doji⭐️Maji⭐️Memorium, Cerberus served as the final boss but didn’t recognize Lucifer as his master. When he tried to attack Satan, Lucifer’s wrath comes force and is able to dominate him.

Since the incident with the grimoire, Lucifer has Cerberus patrol the Mausoleum, a fact the brothers and MC did not know when they tried to retrieve the grimoire in order to lure Lucifer out of being under house arrest.

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.