Mannish Boy

The otherwise unnamed Mannish Boy is a minor antagonist featured in Part III: Stardust Crusaders.

Personality
Mannish Boy is quite intelligent and is able to think like an adult, capable of defending himself from a scorpion and even is seen smoking. When close to people, however, he tends to act like a normal baby to not attract attention. How he came to discover his stand is unknown.

He only shows his true personality when his victims fall asleep close to him, allowing Mannish Boy to use his stand Death Thirteen. Through the stand, Mannish Boy speaks easily, swearing and mocking, while still acting childish, though disturbing. He is also rather sadistic in killing its targets and being prone to fits of maniacal laughter, written as a childish "Hehehe", and boasting, though his over-confidence makes him extremely foolish. Mannish Boy is possibly one of the most sadistic and disturbing characters in the entire series, showing no remorse for human life and enjoying torturing its victims instead of killing them outright, despite being only a baby.

Stardust Crusaders
Mannish Boy was a baby that was given to the Joestar group by a woman, claiming that they needed to find his parents or he will be alone in the world. They agree, without knowing the woman was actually scared of him and that the baby was an stand user. On board of the same plane as the group, Mannish Boy uses his stand to continuously attack Noriaki Kakyoin in his dreams, attacking the others when they also fell asleep. As no one remembers what happened once awaken, everyone deemed Kakyoin crazy when he began suspecting the baby to be a stand user. He eventually found a way to remember the dream by writing a message in his own arm while dreaming. After his stand is defeated and taken control of by Kakyoin, Mannish Boy is told that they are going to leave him in a nearby city where his mother should be around, from there he should return to his parents and stop mocking adults. Kakyoin teaches him that lesson forcefully by mixing some of his poop into his baby food. Naturally, Joseph and Polnareff, who are clueless about this and believe Mannish Boy to be hungry, decide to feed him the contaminated food, forcing Mannish Boy to learn his lesson, while Kakyoin acts like nothing had happened.

Trivia

 * Mannish Boy's catchphrase appears to be him giggling, "hee hee".
 * For some reason, Mannish Boy possesses fangs similar to those of a vampire.
 * Mannish boy is one of only three infant stand users in the series, joined by The Green Baby and Shizuka Joestar.
 * Although Mannish Boy only appears in the PS1 version of JoJo's Bizarre Adventure: Heritage for the Future, his stand Death Thirteen appears in all versions as an NPC that can only be fought by the protagonists when conditions are met.