Mannish Boy is an evil genius baby and Stand user in DIO's service. He tries to murder the Joestar Group inside the nightmare-world of his Death Thirteen.
Mannish Boy's Stand is Death Thirteen, a clown-like Stand which can pull the consciousness of its sleeping victims into a dream-world where it has absolute control and where they cannot summon their Stands, then murder them with impunity.
Precociousness: Demonstrating physical skill also unusual for his age, Mannish Boy is dextrous enough to kill a scorpion with a safety pin. Moreover, he is unusually intelligent for his age, as he's able to speak and have coherent thoughts, despite being only a baby.