Naberius Kalego is one of the prestigious professors at Babylus Academy, a school in the demon world. He is the homeroom teacher of the Abnormal Class, one of Suzuki Iruma's teachers, as well as Iruma's familiar after being contracted to him unbeknownst to Iruma's true nature as a human.
Kalego has short, deep purple, cropped hair with a long fringe and sideswept wing style. He wears a long purple cloak under a short purple shoulder capulet with white trimming and a studded purple belt. His fingernails are black. He has a tall, lean frame and is often seen scowling.
When summoned by Iruma, Kalego appears as a small chick, about the size of a grapefruit, with black, bat-like wings, black, horn-like tufts, and Kalego's distinctive front hair curl. His familiar form is that of a small owl-like creature with bat wings and small black horns.
Kalego has a personality that is somewhat antagonistic in nature. He is derisive, snappy and has a very short temper. He often makes every student around him to keep quiet. He is also rather pessimistic when dealing with his students, giving very little encouragement.
He holds a very strong dislike and detest towards people who are boisterous, brash and inconsiderate of others. A prime example of this is the headmaster's easygoing attitude (thus furthermore hating him because he had done actions like cutting back his pay or forcing Kalego to use a cute study guide to demonstrate to the students how to summon familiars). He similarly holds those who he feels are troublemakers or have easygoing trouble-making go at their pace personalities such as disliking Clara immediately after meeting her and seeming to dislike the Abnormal class whom he teaches.
While not an overly cruel person, he is at best an unpleasant person to be with. When he strongly dislikes someone, while he won't go out of his way to make their life as miserable as possible, he is also not kind enough to help them when they are in trouble. He seems disgruntled to have to be the teacher for the abnormal class and doesn't seem to care for their safety. For example, when Sabnock ignores his warnings during a race and heads to the direction where the Guardian of the valley is, rather than try to help him, he just sits back and erases Sabnock's name from his list of students assuming he will die, saying it meant less work for him. He similarly did the same to Iruma when he couldn't find Iruma through his demon cameras and simply assumed him dead rather than try to find him. He is also sadistic, taking enjoyment when those he dislikes are embarrassed or put in unfavorable positions, such as when Iruma and Sabnock are forced to kneel after they complete the race, he condescendingly laughs at them for their misfortune.
Kalego can also be unreasonable. When he makes deals with someone while he won't outright break the rules or contract, he'll try to find loopholes in his favor or make unreasonable demands and requests. For example, he dislikes Iruma as he is the headmaster's grandson and stated if Iruma made one mistake during the familiar summoning exam he would fail him. Similarly, when Iruma demands a change of classrooms for the Abnormal class (as their class is next to the garbage disposal and their class is in horrible condition) he agrees on the condition that the Abnormal class get approval from the faculty in three days. When the Abnormal class, against all odds, does manage to get all of the signatures, he corrects them stating he meant every employee of the school counts as faculty not just the teachers ( a dirty move that he claims he didn't elaborate on because they never asked) but he lost as Iruma's kindness charmed the rest of the employees into giving their signatures.
He is not a villain or antagonist, however, as he, despite often losing his temper, never gives out cruel punishments, or attempts to punish them for something they didn't do. Despite these negative qualities, he does show a mild ability to compromise. Although he did use an unfavorable loophole to try to reject the classroom change of the Abnormal class, he told them he would at least try to fix their classroom in recognition of their hard work. He also has an odd friendship with fellow teacher Balam whom almost everyone else avoids due to his scary appearance, implying that while he is judgmental of others he doesn't judge others based on appearances alone (though he does unfairly judge other people based on his own perceived biases such as towards Iruma).
Despite this incriminating antagonistic personality, Kalego also known how to be terrified, which is a very contrast of his overall behavior. This example would be his attitude towards Opera, his former upperclassman when they went to the same school, and everytime both are around Kalego's pride and demeanor constantly changes to whatever Opera wishes for him to do, or to be, much on his liking.Aside from distress from Opera, Kalego has also shown a contrasting goodness of himself. He has a lot of integrity as a teacher, and even with his usual antics, as a teacher himself he doesn't back down his responsibilities and in truth he studies and learns about this students in great detail. In fact, Kalego himself wrote the school's guidebook for its teachers. He is also extremely attentive to his students, keeping individual notebooks full of observations for each of them and doing his best to give them each sincere praise and helpful feedback during his home visits. At times, some of his irritation toward his students seems to stem from concern or frustration at them not reaching their own potential. While some of his actions may seem harsh or spiteful, he generally always tries to impart a lesson, whether or not it is a fair one.
Barrier Magic: Kalego has the ability to summon barriers, disrupting any obstacle that blocks or attacks on his field of view. He is able to cast this almost instantly with ease.
Conjure Objects: Kalego can manifest items to his hand, or by snapping his fingers, conjure them at any time. He has done this for simple tasks such as bring to mind books, or let items appear out of thin air.
Unnamed One-Touch Destruction Magic: Using his index finger, presumably he then touches a specific part of place of the target, and that part would then be blown to pieces. Lord Sullivan states that his magic is not allowed.
- Cerberus Butte: A three-headed demon beast with gold-colored skin that has a massive strength, eliminating anyone on their way at will.
Kalego holds a massive grudge against Iruma for multiple reasons – causing a scene at the opening ceremony, humiliating Kalego by summoning him as a familiar, being the grandson of his willful boss, and just generally being an unpredictable troublemaker, whether or not that is Iruma's intention. Despite this, some of Kalego's irritation seems to come from a place of concern. He repeatedly warns Iruma not to take it for granted that he can manage any crisis on his own, and to rely more on the adults around him, namely Kalego himself. He even seems confused as to why Iruma doesn't summon him as a familiar in dangerous situations, despite Kalego's own proclamation that he would never truly be Iruma's familiar. Iruma, in turn, seems deeply touched whenever Kalego shows concern for him. During his home visit to the Sullivan household, Kalego spoke his unbiased and honest opinion of Iruma as a teacher, praising Iruma's academic progress and saying that he believed that Iruma's will to overcome any challenge was his most precious talent.
Iruma's actions during his wicked cycle hint at some latent resentment toward Kalego, as Momonoki's signature is the only one whose signature is obtained by underhanded means – namely, stealing Kalego's personal diary and using it to extort her. However, for the most part, he seems to respect Kalego as a teacher and appreciates him as an adult who cares and looks out for him. During the Harvest Festival, Iruma hallucinates many of the people who he cares about turning their backs on him for being human, including Kalego, which deeply upsets him.
Balam and Kalego were classmates together at Babylus and appear to be long-time friends, much to the shock of Kalego's students. The two are on a first-name basis and Kalego is notably more patient with Balam than with most people, even when Balam compulsively hugs and pets Kalego in his fuzzy familiar form. In a volume extra, Kalego even shows a deep understanding of his friend's insecurities about eating with other people, and leaves a party to go order room service for himself and Balam to eat together by themselves.
Opera was Kalego's upperclassman in their school days at Babylus, and their antics seem to have scarred Kalego for life. Opera still greatly enjoys humiliating Kalego at every opportunity, though they act as though it's just affectionate ribbing and makes the dubious claim that they were very close in their school days. Kalego, on the other hand, avoids Opera whenever possible, even going as far as using Iruma as a literal human shield during his home visit to the Sullivan house. Little is known about their school days, other than Opera refers to themselves as Kalego's senpai who badgered him in school and made him carry all their bags.
Kalego strongly dislikes Sullivan, who embodies all three traits that Kalego hates most – obnoxious, brash, and inconsiderate of others. On top of this, Sullivan often seems to single out Kalego for fun, doing things like cutting his pay on a whim and creating annoyingly cutesy instructional tools just to force Kalego to use them in class. Sullivan hiring Opera as his personal butler may also contribute to Kalego's grudge. In spite of Sullivan's antics, he seems to appreciate Kalego's abilities as a teacher, naming him as interim headmaster when Sullivan was arrested, and proudly declaring that he knew he had made the right choice to assign Kalego to the misfit class as he showed off Kalego's thorough notes on Iruma's progress.
Kalego deeply dislikes Robin, disagreeing fundamentally with his approach as a teacher and perhaps also simply holding a grudge against him for the humiliation that he suffers as Iruma's familiar in Robin's class. On top of this, Robin is exactly the type of unwittingly obnoxious person that Kalego hates the most. Oblivious to both Kalego's identity as Iruma's familiar and his obvious hatred toward him, Robin seems to admire Kalego a great deal as a teacher and constantly pushes his buttons by trying to be helpful, egged on by the other faculty members. Nevertheless, Kalego seems to have some respect for Robin's abilities as an archer and even a teacher, as he assigned Robin to be Iruma and Shax Lied's personal tutor in preparation for the Harvest Festival.
- In demonology, Naberius (also spelled as Naberus, Nebiros, Cerberus, and Cerbere) is a Marquis of Hell, having 19 legions of demons under his command. He can make men cunning in all arts (especially in rhetoric) and sciences, restore lost dignities and honors (also procure the loss of them according to some authors) and teach the art of gracious living. He is depicted as a crow or a black crane, and having a raucous voice but presents himself as eloquent and amiable. Concerning his name, there is an unclear association with the Greek Cerberus (Three-headed hound of the Underworld).
- Nevertheless, Naberius still often depicts as a three-headed dog or a raven. He has a raucous voice but presents himself as eloquent and amiable, as well as teaches the art of gracious living. In Johann's theory (Johann Weyer) in 1583, Naberius and Cerberus are considered to be the same demon.
- He likes to collect and care for cacti, as mentioned in chapter 68 to Clara's mother.
- In a lot of scenarios, Naberius Kalego would often exclaim the word Silence (静か Sizuka).