Naberius Callego is a highly prestigious professor at Babyls Demon School, serving as the homeroom teacher of the Misfit Class, a group of unusual and powerful students. He is a strict, short-tempered, and intimidating figure who demands discipline and excellence from everyone around him. He holds a strong disdain for demons with lazy or easygoing attitudes and rarely offers praise, preferring to scold and push his students toward improvement through rigorous training and harsh criticism. Despite this outward severity, he demonstrates a deep sense of responsibility and an analytical mind; he writes personalized notebooks for each student in his class, cataloging their strengths, habits, and areas for growth, and provides them with precise advice on how to advance. His teaching methods are rooted in a belief that personal strength and merit must be earned through effort.

Callego comes from the noble Naberius family, a lineage known for its discipline and magical power. He was trained from a young age and later became a student at Babyls himself, where he was classmates with Balam Shichiro and Opera, forming a notable trio of future elite demons. His history with Opera in particular is marked by a mix of rivalry and mutual respect, often leading to comedic situations where Opera easily outmaneuvers him. After graduating, he rose to become one of the most respected and feared faculty members, eventually taking on the role of head teacher and, when necessary, acting as chair-demon in the absence of Principal Sullivan.

A key and ongoing source of frustration for Callego is his accidental familiar contract with Iruma Suzuki, a human attending Babyls under false pretenses. Because Iruma is human, the summoning ritual went awry, binding Callego as Iruma’s familiar rather than the other way around. Whenever Iruma summons him, Callego transforms into a small, fluffy white bird-like creature with bat wings and horn-like tufts, retaining his grumpy personality while being forced into an adorable and humiliating form. This transformation is a running gag in the series, and despite his intense annoyance, Callego reluctantly fulfills his duties as a familiar, offering protection and support to Iruma in combat situations.

In terms of abilities, Callego is ranked at Khet, the eighth rank, making him the second-highest-ranked demon at the school after Sullivan. His signature power is the summoning of Cerberion, a massive three-headed dog that serves as both an offensive weapon and a guardian for the school. He is also highly skilled in magical combat and has written the Babyls student handbook. His analytical prowess extends beyond teaching; he often makes sharp, insightful observations about situations and people. Despite his cold demeanor, he has a dry, sarcastic sense of humor and personal hobbies such as caring for cacti and enjoying music.

Over the course of the story, Callego’s character sees gradual development, though he remains fundamentally strict. He begins to acknowledge the potential of the Misfit Class, especially Iruma, whom he initially views as a frustrating anomaly. While he never openly expresses warmth, his actions show a growing, if reluctant, care for his students’ well-being and success. His relationship with Iruma evolves from pure exasperation to a begrudging mentorship, and he unknowingly plays a crucial role in Iruma’s growth. Through his interactions with other faculty and his own past, Callego is portrayed as a demon who, despite his harsh exterior, is deeply principled and honorable, committed to the standards of his position and the future of his students.