Spoiler!It is eventually revealed that she is Joseph's long-lost mother, despite her youthful appearance.
Her real name is Elisabeth Joestar and she is the infant Jonathan Joestar saved at the end of Phantom Blood and gave to his widow Erina to raise. She was subsequently adopted by the Ripple user Straits, and learned the art of the Ripple from him. As a young adult, Elisabeth eventually married George Joestar II, and they had one child. After George had been killed by a vampire, Elisabeth murdered her husband's killer and burned him to ash, but he happened to be a high ranking military official. In order to avoid retaliation, the Speedwagon Foundation helped her to erase all evidence of her existence.
She is gravely wounded in her one-on-one struggle with Kars, but manages to survive. She later tells Joseph her relation to him personally, at his own funeral. She eventually remarries.
Seiyuu: Tanaka Atsuko