666 (六百六十六 ) is a specific number that is most prominent and an important aspect in the Mairimashita! Iruma-kun's Demon World Universe. This includes either more than or less than six-hundred sixty-six as long as the number 6 is present.
In superstitions, the number 666 is often affiliated with devils or demons, as well as signifies evil. It is commonly interpreted as a number to invoke Satan.
The association of 666 with evil originates from a passage in the Book of Revelations in the New Testament of the Bible. As it reads in the King James Version (13:18): Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.
The idea of the “number of a man” or “beast” may refer to the practice of gematria in Jewish numerology, where every letter of the Hebrew alphabet corresponds to a number (e.g., aleph=1, bet=2, etc.), and words and names correspond to the sums of these numbers, which is then interconnects with the series' Demon Ranking System.
The following details the mention of the numbers 666, or in some instances, the use of 6.
- It serves as Lord Sullivan's finger pose when he took a picture with Iruma Suzuki's Entrance Ceremony.
- Babylus Demon School has a total of 666 students enrolled.
- The formal entrance ceremony for the Babylus Demon School starts at 6:06am. (It is likely that it is 6 seconds in total, making it 6:06:06 am).
- Babel Tower has a total of 666 floors.
- Kuromu's Akudol concert had garnered 666 views on Makuboru Dome.
- The Royal One's chair of which the Demon King stood upon has 666 number inscribed on it.
- Azazel Ameri survived her Harvest Festival in 6666 minutes; of which Harvest Festival spans as the time limit.
- Eiko Oni survived the Harvest Festival, garnering a total of 666 points.
- 6th of June (6/6) serves as the Day of the Devil. 
- The Hell Dance compromises of 6 dancers of which all members are to synchronize dancing for a long period of time, and has to go through an imaginable agony doing it.
- Amduscias, the Legendary Sound Demon and a former member of the 13 Crowns, once stalled enemy troops for 666 hours with a concert.
- The Abnormal Class' scoreboard from their performance on the Music Festival is 666. (Though it was initially 661, the score 1 was changed into a flat note symbol, interpreted as 66♭).