Star Birthmark

The Star Birthmark (星のアザ) 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 first in Stardust Crusaders and became largely important in Stone Ocean. In the Anime the birthmark is shown far earlier on Jonathan Joestar and Joseph Joestar. It reappears in the Steel Ball Run universe in Jojolion.

History
While the origins of the Star birthmark is left unknown, starting from Phantom Blood and onwards every blood member of the Joestar family carries the birthmark. During the climax of Phantom Blood, After killing and taking control of Jonathan Joestar's body, Dio Brando inherits his birthmark but is confined to a coffin at the bottom of the sea for the next century. In the year 1983 DIO is released from the coffin and spends the next few years impregnating multiple women and fathering multiple children, most notably Giorno Giovanna,Vento Aureo's protagonist. DIO 's other sons Ungalo, Donnatello Versus and Rikiel also posses the birthmark.

The birthmark was first introduced by Joseph Joestar to Jotaro Kujo realizing that DIO had stolen Jonathan Joestar's body and his birthmark due to his Stand's photograph. 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 the events of Stone Ocean following the conflict between Jolyne Cujoh and Enrico Pucci, Pucci fuses with the Green Baby causing both him and Weather Report to receive the birthmark as the Green baby partially originated from 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 birthmark, helping him identify Jolyne as reincarnating 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 Joestars, or members of DIO's progeny.

Steel Ball Run Universe
In the new universe, The birthmark shares similar unknown history in the Joestar family other than the fact that seemingly all members of the bloodline possess it. Johnny Joestar's is never visually shown with it 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 Josuke Higashikata's birthmark is revealed to be from Yoshikage Kira due to a Locacaca fusion with Kira's body. Kira's sister Kei Nijimura is also revealed to have the birthmark. For Josuke, the birthmark factors in his Stand ability along with the with the Spin, as the bubble comes out through it.

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

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.

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.
 * Josuke Higashikata's birthmark is only visibly depicted in the Diamond is Unbreakable anime.
 * Josefumi Kujo was originally depicted with a birthmark but it was later revealed to be a mistake, and was removed in recent reprints of the manga.