Diablo Personality: A Loyal and Whimsical Demon Butler

Updated: Monday, December 22, 2025

MBTI

INTJ

Enneagram

3w4

Alignment

Chaotic Evil

Diablo is a major supporting character in the anime, manga and light novel series That Time I Got Reincarnated As A Slime. He is one of the seven Primordial Daemons, serving as the origin for the “black” demons. Originally a powerful archdemon, he was summoned by Rimuru, whom he pledges his loyalty to, evolving into a demon duke after being named. He now serves as Rimuru’s “secondary secretary”.

Diablo's Personality

Diablo is a loyal, whimsical, calculative, sadistic, and respectful demon. He is devoted to serving Rimuru and fulfilling his every wish, even if it means going against his own nature or preferences. He is also very playful and enjoys teasing or manipulating others for his own amusement, especially those who are close to Rimuru. He is capable of devising elaborate schemes and strategies to achieve his goals, often surprising his enemies and allies alike. He is also very cruel and merciless to those who oppose or insult Rimuru, and does not hesitate to torture or kill them in the most excruciating ways possible. He is, however, respectful to those who are powerful or worthy of his admiration, such as Shizu or the other Primordial Daemons.

Diablo’s origins are shrouded in mystery, as he is one of the oldest and most ancient beings in existence. He was born from the primordial chaos, along with the other six Primordial Daemons, each representing a different color and aspect of the world. Diablo represents the color black and the aspect of darkness, and is the leader of the black-colored daemons. He has lived for countless eons, witnessing the rise and fall of many civilizations and worlds, but never finding anything that could satisfy his boredom or curiosity.

One day, he decided to answer a demon summoning on a whim, and encountered a human girl named Shizue Izawa, who was possessed by the fire spirit Ifrit. Diablo was intrigued by her power and courage, and decided to fight her for fun. However, he was repelled by her anti-magic mask, which had an incomprehensible nature to him. He was impressed by her strength and offered to spare her life if she became his master, but she refused. Diablo respected her choice and left, but not before giving her his name and telling her to call him if she ever changed her mind.

Many years later, he received another summoning, this time from a slime named Rimuru Tempest, who had inherited Shizue’s mask and will. Diablo was overjoyed to see the mask again, and felt a strong connection to Rimuru, who had also absorbed Shizue’s memories and feelings. He immediately pledged his loyalty to Rimuru, and asked him to give him a name. Rimuru named him Diablo, after the Spanish word for devil, and Diablo evolved into a demon duke, gaining a new appearance and power. He was ecstatic to serve Rimuru, and vowed to never betray him or disobey his orders.

Diablo quickly became one of Rimuru’s most trusted and loyal subordinates, serving as his secondary secretary and executive. He also became the leader of the Black Numbers, a group of black-colored daemons who followed him and Rimuru. He was involved in many of Rimuru’s major events and battles, such as the war against the Farmus Kingdom, the awakening of Rimuru as a true demon lord, the confrontation with the Western Saints Church, and the alliance with the other demon lords. He always acted in Rimuru’s best interest, and was willing to do anything to protect him and his friends, even if it meant sacrificing himself or going against his own morals. He also developed a friendly rivalry with the other Primordial Daemons, especially Testarossa, who often competed with him for Rimuru’s attention and favor.

Diablo’s personality did not change much after becoming Rimuru’s servant, but he did learn to respect and care for others more, especially those who were important to Rimuru. He also became more playful and cheerful, often joking or teasing others, and showing genuine emotions such as happiness, anger, or fear. He still retained his sadistic and cruel side, but he only used it against Rimuru’s enemies or those who deserved it. He also became more obedient and humble, always following Rimuru’s orders and wishes, and never questioning his decisions or authority. He also accepted Rimuru’s kindness and generosity, and was grateful for his trust and friendship.

Relationships

Rimuru Tempest: Diablo’s master, lord, and friend. Diablo is utterly devoted to Rimuru, and considers him the most important person in his life. He is always eager to please him and fulfill his every wish, even if it goes against his own nature or preferences. He is also very protective of him, and will not tolerate anyone who harms or insults him. He is loyal to him to the point of fanaticism, and will never betray him or disobey his orders. He also loves him as a friend, and enjoys spending time with him and talking to him. He respects him as a powerful and wise leader, and admires him as a kind and generous person. He is also very possessive of him, and does not like to share him with others, especially other daemons. He often competes with them for Rimuru’s attention and favor, and tries to get closer to him by any means necessary. He is also very jealous of anyone who has a deeper or more intimate relationship with Rimuru, such as Shizu, Shion, or Milim. He sometimes tries to sabotage or interfere with their interactions, but he usually fails or gets scolded by Rimuru. He also respects Rimuru’s choices and feelings, and does not force his own on him. He is content with being his servant and friend, and does not seek anything more from him.
Shizue Izawa: Diablo’s first master and friend. Diablo met Shizue when he answered her demon summoning, and was impressed by her power and courage. He offered to spare her life if she became his master, but she refused. He respected her choice and left, but not before giving her his name and telling her to call him if she ever changed her mind. He never forgot about her, and hoped to see her again someday. He was saddened to learn that she had died, but was overjoyed to meet Rimuru, who had inherited her mask and will. He felt a strong connection to Rimuru, who had also absorbed Shizue’s memories and feelings. He considered Rimuru as his new master and friend, and also as a part of Shizue. He cherished Shizue’s memory, and was grateful to her for introducing him to Rimuru. He also wanted to honor her legacy, and help Rimuru achieve her dream of peace and coexistence.
Testarossa: Diablo’s rival and friend. Testarossa is one of the other Primordial Daemons, representing the color red and the aspect of flame. She is also one of Rimuru’s subordinates and executives, serving as his third secretary. She and Diablo have a friendly rivalry, often competing for Rimuru’s attention and favor. They also have different personalities and opinions, and sometimes clash or argue with each other. However, they also respect and care for each other, and work together for Rimuru’s sake. They are both loyal and devoted to Rimuru, and will not hesitate to cooperate or support each other when needed. They also enjoy teasing or challenging each other, and have a mutual understanding and trust. They are also aware of each other’s feelings for Rimuru, and sometimes try to sabotage or interfere with each other’s plans. However, they also acknowledge that Rimuru has his own choices and feelings, and do not force their own on him. They are content with being his servants and friends, and do not seek anything more from him.

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