Shizuka Joestar

Shizuka Joestar is a baby found by Joseph Joestar in Part IV: Diamond is Unbreakable.

Appearance/Personality
Shizuka is made visible thanks to being covered by clothing, heavy makeup and dark glasses. Still, should she find herself very frightened and distressed, the power of her stand will increase, making her completely invisible even then. Despite her prevalent use of the stand power, it is unknown if and highly unlikely that she is aware of her ability

Diamond is Unbreakable
For unexplained reasons Shizuka was a baby lost in Morioh, probably because her stand power Achtung Baby, which can turn herself and her close surroundings invisible, including the ground, turned her invisible to her parents.

Upon discovering her, Joseph Joestar and Josuke Higashikata are thrown into a wild chase to find her, during which Joseph proves his worthiness to Josuke by slitting his wrist so that his blood would be able to track her location. From then on Josuke saw his father as more than just any "old man" and instead recognized him as someone willing to place himself in danger for the sake of protecting an innocent life.

Shizuka helped Rohan Kishibe when he was attacked by Ken Oyanagi. To win the last rock-paper-scissors match, Rohan used his Heaven's Door in Shizuka to make her force Ken's hand to use rock, allowing him to win the match no matter what Ken decided to choose. She was held hostage by Ken, but Rohan convinced him to let her go in exchange of another rock-paper-scissors match.

For the rest the series, Shizuka remains mostly carried by Joseph, who had adopted her into his family despite risking the wrath of his wife Suzie Q suspecting her to be another illegitimate child of his. Her real parents were never found.

Trivia

 * Her existence was either forgotten or ignored by Araki, it could also be possible that her stand had no particular use for battle and the Joestars preferred her to stay at their house and live a calm happy life there.
 * Shizuka is one of the only three infant Stand-users, joined by Mannish Boy and The Green Baby.