Hachi, whose full name is Fuuma Hachi, is a central character who serves as the loyal apprentice and assistant to the phantom thief Joker. His background is rooted in tradition and a sense of failure, as he originates from the ninja village of Fuuma, where he was born as the eighth child among ten siblings. Despite attending and graduating from his village's ninja school, he was considered completely inept at the ninja arts, earning him the derogatory title of the biggest ninja failure seen in a decade. His life took a dramatic turn when he was sent on a mission to recover the Dragon Gem, a treasured guardian statue stolen from his village by the wealthy Mr. Kaneari. At Kaneari's mansion, he crossed paths with Joker, who was targeting the same treasure. Hachi was immediately captivated by Joker's incredible skill and showmanship, and he began persistently begging to become the phantom thief's apprentice. Although Joker initially refused, Hachi followed him back to the airship, the Sky Joker. There, he proved his worth not through thieving ability, but by cleaning the ship's disastrously messy kitchen and cooking a meal of curry, which immediately won Joker over.

In terms of personality, Hachi begins as a clumsy, anxious, and self-deprecating crybaby, plagued by low self-esteem from his years of being labeled a failure. However, as he trains under Joker, he gradually develops into a more confident and competent individual, a change that is particularly noticeable to those who knew him from his past. Despite his growing confidence, he retains a compassionate and caring nature, often acting as a moral compass for the more reckless and lazy Joker. He is a self-proclaimed clean-freak who takes it upon himself to maintain the tidiness of the Sky Joker, managing all the housework, laundry, and even tending to a rooftop garden for fresh produce. He is openly critical of Joker's slovenly habits and impractical schemes, and he is unafraid to voice his opinions. Hachi also has a strong sense of justice and deep-seated loyalty, which runs so deep that he has been known to take a bullet for his master. He has a few notable fears, particularly a strong aversion to ghosts and the supernatural, and he adores cats, unlike Joker. His motivations are simple but powerful: to become a great phantom thief and to support Joker in every possible way. He finds a sense of purpose and family with Joker that he never had in his home village, and he is driven to constantly improve his skills to repay the faith Joker placed in him.

Within the story, Hachi's role is multifaceted. He is primarily Joker's partner and apprentice, responsible for logistics, cooking, and emotional support, but he frequently proves instrumental in their heists. His cleverness and problem-solving skills often help get them out of tight spots, and he learns quickly, applying tricks from mystery novels or lessons from Joker in the heat of the moment. His key relationships are central to his development. His bond with Joker is the most significant, evolving from an annoyance to a deep, familial partnership where they would risk their lives for each other. He also maintains a friendship with Hyakkimaru Koga, a prodigy ninja from his childhood who protected him and taught him techniques; Koga is one of the first to acknowledge how much Hachi has changed. He has a friendly rivalry with Dark Eye, the assistant of the phantom thief Spade, which is complicated by his innocent crush on Ai, the eldest member of the idol group Shuffle Sisters, who is secretly Dark Eye.

Hachi undergoes a significant development from an insecure outcast to a reliable and skilled phantom thief in his own right. He learns to weaponize his perceived weaknesses, such as using his reputation as a failure to lower an enemy's guard. In future timelines, his evolution is complete, as he becomes a full-fledged phantom thief who takes on the responsibility of training Joker's son, J, while Joker himself explores space. In terms of abilities, Hachi initially struggles with basic ninja skills but eventually masters techniques like the Flying Squirrel glide, Substitution, and Hiding. He uses a variety of tools, including shurikens (which he learned to throw with precision after a single lesson), a grapple hook, smoke bombs, and a small sword on his back. His most unique tools are the special gums created by Silver Heart: Image Gum, a pink gum that allows him to transform his appearance into any object or person he has seen, and Balloon Gum, a green gum that allows him to float through the air. However, his most celebrated ability is his world-class cooking, particularly his curry, which Joker has declared the best in the world. In addition to his crime-fighting skills, he has practical talents for gardening, ceramics, and all forms of housework, making him an indispensable and well-rounded character.