Jotaro Kujo first appears in the narrative of Diamond Is Unbreakable as a twenty-eight-year-old man, having aged ten years since his teenage journey across the world. His background establishes him as the grandson of Joseph Joestar and a marine biologist by profession, who travels to the coastal town of Morioh with a specific purpose. He arrives to locate Josuke Higashikata, the illegitimate son of his grandfather, in order to inform the young man of his inheritance and to warn him about a rising threat. Jotaro also carries the mission of investigating and securing a bow and arrow capable of creating Stand users, which he believes to be the source of the recent disturbances in the town.

His personality in this part of the story has matured from his high school days, though he retains his signature stoic and unflappable demeanor. He is a man of few words, often expressing himself through the catchphrase good grief, and he maintains a perpetually calm and analytical disposition even in the face of danger. Beneath his tough and sometimes intimidating exterior lies a fierce sense of justice, loyalty, and a gentle heart, particularly towards his family and allies. He has little patience for fuss or trivial matters, yet he is highly perceptive and intelligent, quickly assessing situations and opponents. His love for his family, including his mother Holy and his grandfather Joseph, is deep, though he rarely shows it openly, believing that his actions should make his feelings obvious to those who know him.

Jotaro's primary motivation in Morioh evolves over the course of the story. Initially, his goal is the practical one of managing the Joestar estate and locating the dangerous Stand-creating arrow. However, as he and Josuke encounter more Stand users, his focus shifts to protecting the town's innocent residents. The discovery of a methodical serial killer operating in Morioh, Yoshikage Kira, becomes his central objective. Jotaro is driven to stop Kira not only because of the threat he poses to Josuke and his friends, but because Kira represents the kind of remorseless evil that Jotaro has dedicated himself to opposing.

His role in the story is that of a mentor, an elder ally, and a powerful strategic asset. Unlike the previous part where he was the leading protagonist, in Diamond Is Unbreakable, Jotaro serves as a guide and guardian to the younger Josuke. He provides crucial knowledge about Stands and the legacy of the Joestar family, often intervening at critical moments to turn the tide of a battle. He is the anchor of experience for the group, which includes the inexperienced Josuke, Koichi Hirose, and Okuyasu Nijimura. His presence establishes a direct link to the previous generation's battle against DIO, giving the new threats a sense of historical weight and continuity.

Key relationships define his actions in Morioh. The most significant is with Josuke Higashikata, his uncle despite their similar age. Their dynamic is built on mutual respect, though they have a rivalry typical of close family members. Jotaro trusts Josuke's abilities and judgment, often allowing him to take the lead in fights while remaining ready to provide support. He also maintains a close working relationship with Koichi Hirose, recognizing the young man's potential and reliability. His interactions with Joseph Joestar, now elderly and somewhat infirm, show a softer, more dutiful side as he helps his grandfather adjust to his newfound family ties in Morioh. The relationship with the villain, Yoshikage Kira, is one of hunter and hunted, with Jotaro's methodical investigation eventually cornering the killer.

Jotaro's character development in this part is subtle but significant. He transitions from the hot-headed, rebellious teenager of his youth to a calm, responsible, and paternal figure. He demonstrates greater patience and a willingness to cooperate with others, contrasting with his more solitary approach in the past. While he still possesses an intimidating aura, he has learned to channel his intensity into protection rather than pure aggression. His experience with loss and battle has made him more guarded but also more determined to prevent others from suffering similar fates. He shows a growing acceptance of his role as a mentor and protector of the next generation of Stand users.

His notable abilities center on his incredibly powerful Stand, Star Platinum. By this time, Jotaro has mastered its most famous ability, the power to stop time, though his limit is roughly two seconds. The Stand itself possesses phenomenal physical strength, blinding speed, and incredible precision, allowing Jotaro to perform feats such as catching bullets or shattering solid objects with a single punch. Star Platinum also has enhanced senses, including powerful eyesight and hearing. Jotaro's greatest asset, however, is his tactical genius; he remains perfectly calm under pressure and can analyze an enemy's ability, deduce its weakness, and formulate a counter-strategy almost instantly. His ability to stop time, even for a brief moment, serves as a decisive trump card that can reverse an unwinnable situation in an instant.