Joseph Joestar appears in Diamond Is Unbreakable as an elderly man at the age of seventy-nine. Born on September 27, 1920, he is a British-American national who stands 195 centimeters tall. By this point in his life, he works as a real estate agent and has long since retired from the front lines of supernatural conflict, though his past as the hero who defeated the Pillar Men remains an integral part of his history.

In this part of his life, Josephs advanced age has significantly affected both his physical condition and his mental sharpness. He is noticeably more frail, walking with a stooped posture and using a cane for support. His mind also appears to have weakened, and he often seems somewhat senile or scatterbrained, though it is occasionally unclear whether this is genuine or merely an act. Gone is the fiery, combative temperament of his youth; the older Joseph is a much calmer and well-intentioned individual who remains willing to risk his life for the sake of others. He retains traces of his old mischievous personality but is far less energetic than he once was.

His primary motivation in Diamond Is Unbreakable is not to battle a world-threatening villain but to reconcile with a part of his past he had long neglected. Years earlier, Joseph had an extramarital affair with a Japanese woman named Tomoko Higashikata, which resulted in the birth of a son, Josuke Higashikata. For sixteen years, Joseph was absent from his sons life, and his arrival in the town of Morioh is driven by a desire to meet Josuke and take responsibility for his actions. His grandson, Jotaro Kujo, facilitates this meeting, having traveled to Morioh to inform Josuke of his lineage and his rightful place in the Joestar family inheritance.

The most significant relationship in this part of the story is the one Joseph forms with his illegitimate son, Josuke. Their initial interactions are strained, as Josuke harbors understandable resentment toward the father who was never present. Joseph, for his part, seems uncertain and awkward, struggling to bridge the gap of sixteen lost years. A key moment in their relationship occurs when the pair encounter an invisible baby who is a Stand user. During the ensuing chaos, Joseph cuts his own wrist and lets his blood spill into a lake to track the baby, explaining quietly that he wanted to look cool in front of his son. This small, selfless act allows Josuke to see a glimmer of the heroic man beneath the frail exterior and begins to forge a genuine, if unconventional, paternal bond between them. Ultimately, Joseph adopts the invisible baby, naming her Shizuka Joestar, demonstrating that his capacity for compassion and his sense of familial duty remain intact.

In terms of his role in the story, Joseph acts as a supporting character. While the primary narrative follows Josuke and his friends as they investigate a series of mysterious murders, Joseph provides a connection to the larger Joestar legacy and its history of supernatural conflict. He travels to Morioh largely to meet his son but also becomes entangled in the hunt for a Stand user that his abilities have helped to identify. He does not participate directly in the climactic battles, instead serving as a source of occasional assistance and comic relief.

Regarding his abilities, by the time of Diamond Is Unbreakable, Joseph is a shadow of his former self. In his youth, he was a master of Hamon, also known as the Ripple, a martial art that uses controlled breathing to generate energy mimicking the power of the sun, which is devastating to vampires and other creatures of darkness. Due to neglecting his training in his old age, he can no longer perform the spectacular feats he once could, and his body has aged normally as a result. However, he still possesses his Stand, Hermit Purple. This Stand manifests as a tangle of thorny vines that can extend from his hands, which he can use to grapple or bind objects and opponents. Its primary function is divination. By destroying a camera or using a television in conjunction with Hermit Purple, Joseph can produce psychic photographs or video footage, allowing him to see distant locations or the current situation of a specific person from afar. This ability to gather information from a distance is his most valuable contribution to the groups efforts in Morioh.