Carletto, known in the original Japanese version as Tarou Kaibutsu, is the crown prince of Monster Land, a hidden realm populated by supernatural creatures. He is the son of the reigning Monster King, who sends him to live on Earth as part of his training. Carletto takes up residence in an abandoned castle or old house in a quiet suburb of Tokyo, which also serves as his kingdom's ambassadorial base on Earth. He is accompanied by three loyal attendants who act as his entourage and caretakers: the elegant but eccentric Count Dracula, the gentle but incredibly strong Franken, and the werewolf chef Wolf.

In terms of appearance, Carletto looks like a small human child, though he is often revealed to be much older than he seems. He is consistently seen wearing a distinctive hat, which he never removes, even while sleeping. His personality is a blend of royal brattiness and genuine heroism. He is often demanding, prone to tantrums when he does not get his way, and can be short-tempered, especially towards his three servants. Despite this capricious exterior, Carletto possesses a deep well of courage, loyalty, and a strong sense of justice. He is driven by a desire to learn about humans and prove that coexistence is possible, often acting as a mediator between the two worlds. His primary motivation is not only to complete his training but also to protect his human friends from malicious monsters and to teach other creatures that evil actions lead to negative consequences.

As the protagonist, Carletto's main role in the story is to serve as an ambassador and protector, resolving conflicts when monsters from his homeland disrupt the human world. He functions as a classic hero, though he is often clumsy and imperfect, and his adventures typically involve confronting a new monster of the week, helping them understand their mistakes, and turning them into friends. His key relationships are central to the plot. His best friend is Hiroshi Ichikawa, a human orphan boy who lives with his older sister, Sis, near Carletto's home. Through this friendship, Carletto learns about humanity while Hiroshi acts as the audience's window into the world of monsters. He also has a girlfriend named Kaiko, who is the daughter of a duke from Monster Land. His primary recurring enemy is Bem, an alien who seeks to steal Carletto’s powers for world domination.

Throughout the series, Carletto experiences significant development as he navigates his dual responsibilities as a prince and a resident of Earth. A pivotal moment occurs when he returns to Monster Land after hearing of his father's apparent illness. During this journey, his friend Hiroshi is petrified after an encounter with the Monster King, forcing Carletto to directly confront his royal duties and the complexities of his homeland to reverse the affliction. This expands his role from a simple mediator to a leader in a time of crisis.

Carletto possesses a wide array of notable abilities that make him a formidable figure. His primary power is metamorphic, allowing him to change his shape, size, and face at will. He can stretch his limbs to great lengths, a power often used for intimidation or pursuit, and can even transform his body into hard materials like diamond or stone. He also wields superhuman strength and powerful mental abilities, including telepathy and telekinesis. For mobility, he frequently flies on a magic broomstick. Despite these immense powers, Carletto has a notable weakness: he is extremely vulnerable to thunderstorms and lightning, which force him to seek immediate shelter.