She is a young woman who can magnetize her victims which can cause nearby metals to become potential death traps for the person magnetized.
Throughout the story she uses her magnetic powers to try and kill two of the heroes, Mr Joestar and Mohammed Abdul. Her attempts to kill them involve trapping them on an escalator, getting them run over by a train, electrocuting them and crushing them between large metals.
She fails to successfully kill them however, and is in the end taken down by her own magnetic powers. This happens when she manages to attract mass amounts of metal onto both men so they are weighed down and crushed.
Seemingly in full control she stands over Joestar to gloat. Joestar begs her to stop the magnetism as he can't handle it any more. Mariah is too busy enjoying being begged for mercy that she doesn't realise it was a trap by Joestar, and he and Mohammed in their states are opposite each other with Mariah in between. The two men's magnetism causes them to attract each other and crush Mariah in between them, leaving her unconscious with broken bones.