Opera is a demon who serves as the security devil and head servant of Lord Sullivan, one of the most influential demons in the Netherworld and the chairman of Babyls Demon School. In practical terms, Opera acts as the butler and caretaker of the Sullivan household, managing the daily routines of the mansion, including laundry, cooking, scheduling, and transportation, while also assisting Sullivan with his official responsibilities and guarding the estate. When Sullivan adopts the human boy Iruma Suzuki as his grandson, Opera's duties expand to include preparing Iruma for school, escorting him to and from Babyls, and helping him adjust to life in the demon world. Because of this position, Opera is one of the few characters who knows that Iruma is human.

Opera's background remains largely mysterious, but a few details are known. Opera knew Sullivan before enrolling at Babyls, and it was Sullivan who arranged for Opera's enrollment. During their student years, Opera was an upperclassman to both Naberius Kalego and Balam Shichiro and gained a fearsome reputation as the Mysterious Top Dog, a powerful senior known for being harsh toward younger students. At the time, Opera was described as having dark curly hair and two horns and was known to wear both male and female uniforms. This description was so similar to Kalego's that many demons mistakenly attacked Kalego when he first enrolled. In the present story, Opera occasionally takes on a teaching role at Babyls, most notably serving as the instructor for the Misfit Class after the Heartbreak arc in order to protect the school while Sullivan and Kalego are away and to help the students grow stronger.

In appearance, Opera has an androgynous look with long red hair, red eyes, and distinctive cat-like ears and tail. Their personality is calm, composed, and largely unreadable, as they usually wear a poker face and rarely display emotion. Iruma has learned to read Opera's mood through their ears, which stand up when Opera is in a good mood and droop when they are not. Beneath the impassive exterior, Opera is sharp, calculating, and dryly sarcastic, with a mischievous streak. They enjoy teasing others, especially Kalego, whom they treat as an errand boy and take pleasure in provoking. Despite this playful side, Opera is always reliable, honest, and fiercely protective of the people they care about. They often try to serve as the voice of reason for the impulsive Sullivan, though their sensible advice is frequently ignored.

Opera's primary motivation is loyalty to Sullivan. Opera has expressed deep adoration and respect for their master, noting that Sullivan taught them how to serve. Their devotion is rooted in mutual trust: Opera believes that by dedicating everything to Sullivan, they will be repaid with the same faithfulness and more. They extend this principle to Iruma, promising to place the same trust in him as long as he continues to live as part of the Sullivan family. This commitment drives them to keep the household stable, protect its members from danger, and support Iruma through practical advice and quiet encouragement.

Within the story, Opera serves as the quiet foundation of the Sullivan household, imposing order on a chaotic environment and acting as both guardian and caretaker for the family. Opera is present in the background of much of Iruma's daily life, but steps forward decisively whenever a threat appears. As Sullivan's direct servant and bodyguard, they also carry considerable authority, including influence within Babyls. Their relationship with Sullivan is one of deep mutual reliance, with Sullivan depending heavily on Opera's competence and judgment. Their relationship with Iruma evolves from that of a distant, efficient caretaker into something warmer, and they join Sullivan in doting on the boy in their own understated way. Their dynamic with Kalego is defined by their shared history as senior and junior, and Kalego remains one of the few characters genuinely afraid of Opera, a fear Opera clearly enjoys.

In terms of ability, Opera is among the strongest demons depicted in the series, although their official rank has never been revealed. They operate at a level comparable to Kalego and Balam, both of whom hold the rank of eight, and some consider Opera to be even higher. Opera's combat style relies heavily on physical power rather than magic, and they possess tremendous superhuman strength, enough to tear through powerful magical attacks with their bare hands. They also display great speed over long distances, fast reflexes, keen eyesight and sense of smell, and extreme precision in striking targets. Opera is a quick and pragmatic thinker, able to assess a situation and respond without wasted motion. They manage demanding household and defensive duties as well, including maintaining the mansion's barrier systems, and they have demonstrated skill with spatial movement and perception-based magic. As a teacher, they made use of non-verbal magic and unconventional strategies to train the Misfit Class quickly and effectively.

Opera's development across the series is gradual but meaningful. They begin as a composed and somewhat distant servant who is concerned about Sullivan's decision to adopt a human grandson, but over time they become a devoted protector and supporter of Iruma, showing warmth and trust where they once showed only efficiency. Their mysterious past is explored further in a spin-off focused on Kalego, in which Opera appears as a central character, and within the main story they grow beyond the role of household servant by taking on responsibilities at Babyls itself. Through all of this, Opera remains defined by quiet competence, fierce loyalty, and an unshakable commitment to the family they serve.