Star Birthmark

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

It was introduced in Stardust Crusaders and becomes important again in Stone Ocean. The anime incorporates the birthmark earlier, depicting Jonathan Joestar and Joseph Joestar with it.

Original Universe
The origin of the Joestar birthmark is unknown, but within Phantom Blood onward, every member of the Joestar family bears the birthmark. At the end of Phantom Blood, Dio Brando murders Jonathan Joestar and takes over his body. Through this process he inherits the birthmark but rests at the bottom of a sea for a century. Later, in 1983, DIO reawakens after escaping his coffin. He travels around the world and impregnates multiple women with his children, notably Giorno Giovanna, the protagonist of Vento Aureo.



The birthmark is only formally introduced in Stardust Crusaders by Joseph Joestar to Jotaro Kujo. With DIO's reawakening, the special connection he gained through Jonathan's body puts Holy Kujo in danger, so they set out to destroy DIO. A few years before these events, Joseph Joestar has an affair that results in Josuke Higashikata; who as a part of the Joestar family, inherits the birthmark.

In Stone Ocean, the birthmark becomes integral once again. Jotaro Kujo's daughter Jolyne Cujoh faces off against an eccentric priest, Enrico Pucci. During the events of Stone Ocean, Pucci fuses with the Green Baby and gains the birthmark, as the homunculus partially originated with DIO's Bone. His brother, Weather Report also receives the mark; and as twin brothers, they connect and affect one another in the same way DIO does with Joseph, Holy and Jotaro in Stardust Crusaders.

At the end of the story, Emporio Alnino enters the new universe where he discovers Irene, a woman nearly identical to Jolyne, shockingly possesses the mark.

Steel Ball Run Continuity
Like the original universe, the origin of the birthmark is unknown but seemingly all members of the Joestar family bear it. It is unknown if Johnny Joestar or his family have it though.

Later on, in JoJolion, the protagonist Josuke Higashikata bears the birthmark through seemingly unknown means. It is eventually revealed that he inherited this birthmark through a Locacaca fusion of Yoshikage Kira and Josefumi Kujo. Kei Nijimura, Kira's sister, bears the birthmark along with Josefumi, however his connection with the family is currently unknown.

The Joestar birthmark plays into Josuke's usage of the Spin, as the bubble comes out through it.

Trivia

 * Both Jonathan and Joseph Joestar didn't have the Joestar birthmark until Stardust Crusaders, but it was later included in the anime. Josuke also wasn't depicted with the birthmark until the anime.
 * Johnny Joestar is the only JoJo thus far that has not been officially depicted with a birthmark.
 * An artwork in the Hirohiko Araki JoJo Exhibition 2012 shows Lisa Lisa with the birthmark, despite being from a different family and is not biologically related to the Joestar Family.