Welcome To Demon School, Iruma-Kun! is about Iruma, who has to learn how to cope with demons at Babyls while concealing the fact that he is a mere human. Students at the Demon School are taught how to use Bloodline magic and how to survive in the Demon World by the world's most prestigious professors. Among the lessons, Kalego teaches students that summoning a Familiar is an essential step to surviving in the Netherworld.
Familiars are magical creatures that take the form of lower-grade demons who aid demons during battles in Welcome To Demon School, Iruma-Kun. Each Familiar's form depends on the summoner's rank and strength. Also, each Familiar can serve its master for just a year and won't be able to serve the same owner the following year. During the pact, a Familiar is forbidden from disobeying or hurting its master, or else a heavy punishment will immediately strike it. Furthermore, forming a contract with a Familiar requires a few steps to be done successfully. The first step is to draw a circle in blood on a piece of parchment, set one of its corners on fire, and then the smoke will take on a form, and a Familiar will emerge.
5 Kelbie
Kelbie was summoned by Sabnock Sabro in Welcome To Demon School, Iruma-Kun. Kelbie is a mutant beast with the characteristics of both a fish and a horse. He has a horse's head, legs, and body, as well as a fish's fins, scales, and tail. These forms enhance Kelbie's abilities and enable it to stay on land and in the water.
Kelbie has water-based powers and is the same size as the Gorgon snake, which makes it huge compared to other Familiars. Summoning such a strong and big Familiar is not an easy task in Welcome To Demon School, IrumaKun which only means that Sabro is not an easy opponent to face and that all his training has been well-spent. This point can be proven by Balam's reaction when he sees Sabro's and Alice's Familiars and starts questioning if these two are really just first-year students in the Demon School.
4 Gorgon Snake
The Gorgon snake is Alice's Familiar in Welcome To Demon School, Iruma-Kun, and it is known as one of the strongest among them. The winged snake can breathe fire and burns anyone in plain sight. It is distinguished by its massive size, white scales, purplish wings, and tail that is literally on fire.
During the summoning course, Kalego is immediately impressed by Alice's ability to stand still in the presence of the Gorgon snake's fearful aura. Then he proceeds to explain that summoning such a strong Familiar seems natural considering Alice's strong lineage in Welcome To Demon School, Iruma-Kun. The face-off in front of Walter Park Carmine Dragon is enough to showcase the snake's power and how its strength can be enhanced further.
3 Blizzard Wolf & Snow King Fox
Kerori Crocell (Kuromu) can summon two Familiars at once due to her irresistible charms that even animals can't resist. She is one of the few first-year students capable of calling two Familiars simultaneously. Blizzard Wolf and Snow King Fox are among the strongest summoned creatures in Welcome To Demon School, Iruma-Kun, since they are both capable of shooting ice blasts at their opponents.
When they first appear along Kerori, she has to order them not to devour the other creatures since she thinks 'violence is a weapon of the weak.' Also, when Caim convinces the other animals to inhabit the chosen territory, he assures them that it is under Kerori's control, along with her two powerful Familiars.
2 Eggy-sensei (Kalego Naberius)
Eggy-sensei is none other than the prestigious teacher Kalego Naberius, who became Iruma's Familiar due to his ignorance of Iruma's race. As a mere human, Iruma could not form contracts with non-demons. The Parchment used by Iruma had Kalego's seal on it during the contract rituals, so he became his Familiar. Kalego is immediately transformed into an adorable fluffy owl with small bat wings after Iruma completes the Familiar summoning.
The prestigious teacher is considered one of the most powerful teachers in Welcome To Demon School, so being a Familiar with a human is a big advantage since he can overcome every obstacle. Eggy-sensei's calm demeanor throughout the beast's face-off at the water park and the school attack demonstrates his strength as a professor and as a Familiar.
1 Cerberus
Cerberus is arguably the strongest Familiar in Welcome To Demon School, Iruma-Kun. Due to his aggressive aura, every other creature cows in his presence. As a result, Kalego is responsible for the Familiars class. Because Cerberus is bound to the Naberius Bloodline with mana, the family cannot form pacts with other Familiars, making it one-of-a-kind.
Cerberus has effortlessly proven his power in Welcome To Demon School multiple times, like when he bypassed the glass barriers in the school with a simple dash, or when he took down the Mountain Blue beast in Walter Park with only one paw.