Opera functions as Lord Sullivan's primary security demon, residing in his mansion and managing daily responsibilities including laundry, cooking, scheduling, and transportation for Sullivan and his adopted grandson, Iruma Suzuki. Their duties encompass protecting Sullivan's life, status, appearance, and reputation, later expanding to safeguard Iruma after the Akudol Games.

Opera possesses a distinctly androgynous appearance: long braided red hair with a straight-cut fringe, pointed elf-like ears, red cat-like ears atop their head, a black cat-like tail, red eyes, and pale skin. They favor formal attire such as a red vest over a white shirt, a striped tie, and navy dress pants. As a student at Babyls Demon School, they wore the female uniform top paired with male uniform bottoms, contributing to their ambiguity.

Their personality blends severity with dry humor and mischief. While often acting as Sullivan's voice of reason, they are frequently ignored. Opera exhibits a reserved, kuudere-like demeanor, though subtle emotions surface through their cat ears and tail. A more sadistic, playful side emerges around former classmates like Naberius Kalego, whom they bullied during school and continue to tease in adulthood, evoking Kalego's genuine fear. This dynamic stems from Opera's past as the "Mysterious Top Dog of Babyls," a figure who sent 666 adversaries to the infirmary while maintaining anonymity until discovered by Kalego and Balam Shichiro.

Opera's abilities include superhuman strength, demonstrated by effortlessly halting a demonic beast's attack and shattering durable targets; extreme precision, such as splitting arrows midair; and rapid reflexes enabling swift, agile movements even while carrying others. Their combat prowess is highlighted during the Walter Park incident, shielding students from a Panther Rat, counterattacking with devastating kicks, and later collaborating with high-ranking demons to eliminate a merged beast.

Their relationship with Iruma evolves significantly. Initially skeptical of Sullivan adopting a human, Opera later trains Iruma for the Execution Cannonball challenge, adapting methods to overcome Iruma's reflexive dodging. This shared struggle fosters mutual understanding, with Iruma learning to interpret Opera's moods through ear positions. Post-training, Opera develops earned confidence in Iruma, accepting him fully within their protective duties.

As a teacher at Babyls (a role initiated post-Heartbreaker exam), Opera's methods emphasize practical growth. In one lesson, they task the Misfit Class with scaling a magic-repelling pillar using only basic spells and physical strength, deliberately engineering the exercise to force students to develop non-verbal spell-casting. Despite the brutal challenge, Opera displays pride in the class's ingenuity and teamwork.

Opera's gender identity remains intentionally ambiguous. They exclusively use gender-neutral language and attire, responding evasively to Iruma's question with "choose whatever you like." This ambiguity extends to withheld personal details like birthdate and height, reinforcing their preference for mystery. Spin-offs like the "If Mafia AU" depict them maintaining their formidable, enigmatic traits within criminal settings.