Alaindelon Bathin de Emuna, often referred to simply as Alaindelon, is a transdimensional demon with a striking and unmistakable appearance. He is a tall, muscular man with brown hair and brown eyes, and his most notable visual trait is his strong resemblance to the Queen lead singer Freddie Mercury, sharing a similar hairstyle, mustache, and flamboyant attitude. His typical attire consists of casual sweatshirts worn with boxer shorts, though he has been known to wear other outfits like school uniforms or swimsuits on occasion.

In terms of personality, Alaindelon is a highly comical and eccentric individual. He often attempts to carry himself with seriousness, but his inherent uselessness in practical matters and his odd behavior usually undermine his efforts. He is very open about his demonic nature and shows little interest in concealing his powers while in the human world, a trait that sets him apart from other demons. He frequently uses his abilities in plain view of humans, seemingly oblivious to the shock, panic, or disgust he causes.

Alaindelon's primary and most significant ability is dimensional travel. As a transdimensional demon, he can move between different dimensions and teleport across great distances. He performs this by splitting his own body open to allow other people or objects to enter him, effectively carrying them inside his form to a desired location. He often announces this action with a shout of "Transfer!" While he lacks conventional physical strength, he compensates for this with remarkable durability, able to withstand attacks that would fell other demons.

His role in the story begins when he first appears floating down a river, seemingly dead. He is discovered by Tatsumi Oga, and it is from within Alaindelon's body that the infant demon lord Beelzebub emerges, which ultimately forces Oga into the role of Beelzebub's caretaker. After surviving this initial encounter, Alaindelon becomes a recurring presence, frequently appearing to bother or assist Oga and his friend Takayuki Furuichi. He later decides to move into Furuichi's home, having declared Furuichi a noble person.

Alaindelon maintains several key relationships with other characters. He is deeply devoted to his daughter, Angelica, prioritizing her happiness and safety above all else, such as when he is determined to survive a mortal wound to keep a promise to buy her dresses. He holds an intense, idealistic admiration for Furuichi, whom he refers to with great respect, much to Furuichi's own bewilderment and discomfort. With Oga, he has a more pragmatic, though still bizarre, working relationship. Oga generally finds him annoying, but Alaindelon's transport ability proves invaluable, particularly for keeping track of the distance between Oga and Beelzebub to avoid the fatal shock that occurs if they are separated by more than fifteen meters. He also works closely with Hilda, Beelzebub's demon maid, assisting her by delivering items or people and often giving voice to her unspoken thoughts, such as her hidden faith in Oga's abilities.

Throughout the narrative, Alaindelon shows a degree of development in his interactions. While he begins as a mysterious figure who facilitates the core premise, he evolves into a loyal, if unconventional, supporter of the main group. His dedication is especially evident when he rushes to Furuichi's aid upon hearing he is in danger, donning samurai armor and expressing genuine distress at the sight of his injured friend. His role expands from a simple plot device for transportation to a character who participates in reconnaissance, combat support, and provides comic relief, all while demonstrating a hidden layer of heartfelt loyalty to those he considers worthy.