The character known as Joker is the protagonist of Mysterious Joker, a phantom thief who operates on a global scale and is known by the title of the miracle maker. His true name is Jack Jones, and he lived in a mansion with his parents during his early childhood before they were killed in a plane crash orchestrated by his archenemy, Professor Clover. This tragic event is the foundational tragedy of his life and a primary driver for his path as a thief. Orphaned, he was taken in and trained by the legendary phantom thief Silver Heart, alongside two other children who would become his lifelong rivals and partners, Spade and Diamond Queen.

As a person, Joker embodies a complex mix of flamboyant showmanship and lazy domesticity. When performing a heist, he is dramatic, confident, and deeply theatrical, treating every theft as a grand performance for an admiring audience. He thrives on danger and tight spots, stating explicitly that getting into difficult situations is the very reason he enjoys his job as a phantom thief. However, his personality shifts dramatically when he is off the clock. In his personal life, he is extremely lazy, spending his free time rolling around, watching television, reading manga, and playing video games. He hates doing laundry, cleaning, and has a notable fear of cats, which stems from a childhood incident where he was chased by a cat-like creature through a ventilation system. He is also a big eater with a pronounced love for curry dishes, which his apprentice often cooks for him. Joker is not a traditional hero; he operates with a selfish and childish streak, often becoming upset with villains not for their evil deeds but because their actions ruin his show or deprive him of an audience. Despite this, he adheres to a strict personal code, refusing to use killing tools and stating that a phantom thief has no need for them.

Joker's role in the story is that of the central figure around whom all heists and conflicts revolve. His primary motivation is the thrill of the steal itself, seeking out high-value treasures for the challenge rather than for any noble or altruistic purpose. He consistently targets the collections of the wealthy, like Mister Kaneari, and clashes with rival thieves and criminal organizations. His greatest enemy is Professor Clover, the man responsible for the death of his parents, whose violent gang of thieves represents the dark antithesis to Joker's performance-based criminality. Joker's heists also put him in constant opposition to the police, most notably the dedicated but often outmaneuvered Chief Oniyama.

His key relationships are central to both his success and his character development. His constant companion is Hachi, a clumsy but loyal ninja apprentice who acts as his assistant and caretaker, handling the chores Joker despises and providing emotional support. His dynamic with Spade is that of a Vitriolic Best Buds, as they are constant rivals who frequently compete for the same treasures but will join forces without hesitation against a common threat. Diamond Queen, his fellow student under Silver Heart, is another close ally, often joining him on adventures alongside her talking dog, Roko. Silver Heart himself serves as the gruff but caring mentor figure who shaped Joker into the thief he became.

Throughout the series, Joker does not undergo a radical personality overhaul, but his motivations are subtly reinforced and clarified. His backstory as Jack Jones, a boy who lost everything, explains why he lives so fully in the present as Joker, treating each heist as a magnificent show to be enjoyed. His development is often shown through his growing reliability and the trust he places in his team, moving from a purely selfish operator to someone who will fight fiercely for the people he cares about. He also faces rivals who mirror his own abilities, such as Shadow Joker, forcing him to confront and surpass his own techniques.

Joker possesses an array of notable abilities that make him a nearly unbeatable phantom thief. His most versatile tool is Miracle Gum, a special item that allows him to create extremely realistic disguises, allowing him to impersonate anyone from a police officer to a young girl. He is also a master of illusion and misdirection, using a variety of named techniques in combat, including Straight Flash, a blinding light attack, Emblem Fire, and Shooting Star. Beyond these specific tools, his greatest assets are his super smart brain and ability to perform miraculous escapes from seemingly impossible traps, cementing his reputation as the miracle-making phantom thief.