Star Birthmark

The Joestar Birthmark (ジョースター母斑) is a five-pointed, star-shaped mark appearing in the region of the back of the left shoulder on all JoJos or individuals of or in certain ways connected to the Joestar bloodline.

Introduced in Stardust Crusaders; having stolen Jonathan Joestar's body, DIO inherits his birthmark. Fathered after this theft, DIO's sons Ungalo and Rikiel also bear the mark, as well as Giorno Giovanna. The birthmark was first introduced by Joseph Joestar to Jotaro Kujo. With DIO's reawakening, the special connection he gained through Jonathan's body puts Holy Kujo life in danger so they set out to destroy DIO. A few years before these events, Joseph Joestar has an affair with Tomoko Higashikata that resulted in the birth of Josuke Higashikata; who as a part of the Joestar family, inherits the birthmark. Although Josuke's birthmark is only visibly shown in the anime.

In Stone Ocean's events after fusing with the Green Baby, Enrico Pucci and Weather Report also receive the mark as the homunculus partially originated with DIO's Bone; and as twin brothers, they connect and affect one another in the same way DIO does with Joseph, Holy and Jotaro in Stardust Crusaders. Once Emporio Alniño enters the new universe, he discovers that Irene possesses the mark, helping him identify Jolyne as surviving in a new form.

Albeit they haven't been depicted visibly with the birthmark, it may be assumed that George Joestar I, George Joestar II, and Donatello Versus all have the birthmark as well; as they are JoJos, Joestars, or members of DIO's progeny.

In the new universe, Johnny Joestar's is never visually shown although Lucy Steel is well aware of it. His grandson, Joseph Joestar is shown with the birthmark, and his Stand forms out of it. In JoJolion, the protagonist's birthmark factors in his Stand ability, and it is eventually revealed that Yoshikage Kira, Kei Nijimura, and Josefumi Kujo also have the birthmark. Albeit they haven't been depicted visibly with the birthmark, it may be assumed that George Joestar II, Nicholas Joestar, George Joestar III and Holy Joestar-Kira have it as well.

In the Ninth part The JOJOLands, Dragona Joestar is also shown with the birthmark. While unseen, it is assumed that Jodio Joestar and Barbara Ann Joestar have it as well.

Trivia

 * Johnny Joestar is the only leading JoJo thus far not to have shown a mark in any medium. Although, in JoJolion, it was stated he had the birthmark.