Barbatos serves as the impeccably composed butler to Prince Diavolo, the future ruler of the Devildom. He is an ancient demon of considerable power and mystery, first appearing alongside the angelic exchange students Simeon and Luke. His existence is defined by his unwavering loyalty to Diavolo, whom he follows as if he were his shadow, a role he took on after being lured to the Demon Lord's Castle in Diavolo's youth with the promise of a rare tea.

In terms of personality, Barbatos is the picture of calm and refinement. He is a demon of few words about himself, preferring to keep his own counsel and enjoying the secrets he holds. This reserved nature can sometimes be interpreted as cold or distant, a perception that does not trouble him, as he has never been interested in the opinions of anyone other than his master. Despite this serious facade, he possesses a subtle and dry sense of humor, often teasing others by hinting at secrets he will not reveal or leaving tantalizing details to their imagination. His composure rarely cracks, but he has a well-documented temper that surfaces when he feels his lessons or passions are not being taken seriously; on such occasions, his smile remains fixed, which only makes his anger more terrifying, as evidenced when he threw a pen at Mammon for interrupting a lecture on butler etiquette.

Barbatos is highly motivated by his duty to serve Diavolo and to ensure the smooth operation of the Demon Lord's Castle. He is a perfectionist and a micromanager, paying close attention to the most minute details, from creating schedules broken into one-minute increments to observing a fifteen-second reduction in Diavolo’s document-checking time. His role in the story is primarily as a supporting character and a key figure in the Devildom's governance. He is often the one managing events, teaching etiquette, and providing quiet assistance to the main character and the seven demon brothers. He is also known as the greatest pastry chef in all three worlds—the Devildom, Celestial Realm, and Human World—a skill he takes as seriously as his butler duties, visiting markets three times a day for fresh ingredients and growing his own herbs and vegetables.

His key relationships are central to his character. His bond with Diavolo is paramount, defined by deep loyalty and a master-servant dynamic that has lasted for millennia. He also shares a unique connection with the human sorcerer Solomon, holding a pact with him that predates Solomon's pact with Asmodeus. Barbatos entrusted Solomon with his grimoire of his own free will, a significant gesture of trust. With the seven brothers, he acts as a strict but knowledgeable instructor and an object of some suspicion, particularly for Mammon, who is convinced Barbatos has a hidden torture room in the castle. In contrast, he treats the human exchange student, the main character, with genuine kindness, often saving leftover sweets for them and offering subtle guidance, all while maintaining his characteristic teasing demeanor.

Barbatos's development is subtle, as he is already a very old and powerful demon who has served Diavolo for an immense span of time. He mentions that he was never a child, hinting at a mysterious origin that he refuses to discuss. While he appears to excel at nearly everything, he has shown a rare moment of imperfection when struggling with acting, as he had never done it before. His primary notable ability is to see both the past and the future. He can use this power to select from multiple potential realities the one he prefers to become the sole reality, often by opening doorways to any point in time or place, including the Human World and Celestial Realm. This power is so vast that Diavolo expressly forbids him from using it freely. He can also open portals to the dream world, further demonstrating his extraordinary control over time and reality.