Jack from the Power Stone anime is a mysterious and driven character whose full name is Jack Winslow. He is presented as the last surviving member of the noble Winslow family of Sunland, a fact that carries a tragic history. A ship fire, which claimed the life of his mother, is a central event in his past. It is implied that because this incident occurred over a century ago, Jack is likely more than one hundred years old. His physical appearance is notable for its lithe and agile build, standing at six feet three inches while weighing just one hundred twelve pounds. He has black hair that reaches his shoulders and distinctive red eyes, an adult man whose expression is often hidden behind the wrappings on his face.

Personality-wise, Jack operates on a simple moral framework that makes little distinction between good and evil. His primary pleasures are uncomplicated: he has an intense fascination with anything shiny and a love for cutting with his knife. In his hometown of Manches, this manifested as a habit of robbing strangers of their jewelry. This behavior is not born from malice as much as from a naive, almost obsessive desire for glittering objects. In the anime, this obsession is focused entirely on the Power Stones, which he refers to as his lovely. He does not view his actions as villainous but is instead singularly focused on his goal, to the point of being unpredictable and mysterious in his movements.

His core motivation is deeply personal and rooted in the trauma of his past. Jack desperately seeks a specific Power Stone because it resembles a jeweled necklace that his mother once wore. Claiming this stone is an attempt to reclaim a connection to the family and the mother he lost in the fire. This desire is what drives his actions throughout the story, initially casting him as a relentless antagonist. He first appears at an opera house, leaping onto the stage to steal a jeweled necklace from a singer, an act that draws the attention of the protagonist, Edward Falcon. He then leads Falcon on a chase back to his home, a ghost ship, where his true nature begins to be revealed.

Despite his antagonistic role, Jack undergoes significant development through his relationship with Falcon. When Falcon learns of Jack's past and his connection to the Winslow family, he begins to sympathize with him rather than condemn him. Instead of seeing a thief, Falcon sees a broken soul trying to reclaim a lost piece of his history. This empathy creates a complex dynamic where Falcon tries to help Jack. Ultimately, Falcon allows Jack to escape with a Power Stone, but Jack surprises everyone by returning of his own accord. He chooses to use the stone he coveted to aid Falcon in the final battle against the great evil, Valgas. In a decisive moment of heroism, he shields Falcon from a deadly attack, nearly sacrificing his own life to save his former rival. This act cements his transition from a lone wolf obsessed with a relic to a true friend accepted into the group.

In terms of abilities, Jack is a formidable and unorthodox fighter. His fighting style is described as agile, mysterious, and unpredictable, relying on rapid attacks to disrupt an opponent's rhythm rather than raw power. He possesses a unique movement style, crawling on the ground in a way that allows him to evade many incoming attacks. His ultimate potential is unlocked through the Power Stones, which trigger a transformation known as the Power Change. In this state, he becomes the Mad Clown, his body taking on a distinct purple hue in the anime, and gaining access to enhanced special attacks. These include spinning into a hovering slash known as Killer Dance and summoning a barrage of homing blades with an attack called Misery Rain. Following the final battle, Jack does not return to a normal society but chooses to remain in the mysterious Dark Land, living among the animals, suggesting he has finally found a place where his unconventional nature belongs.