Margaret of York
Margaret of York (1497-1560) was the only child of Richard III and Joanna of Castile, his queen. In 1512, at age sixteen, she married James Stewart, Duke of Rothesay and heir to the throne of Scotland, who was invested as the Duke of Kent. Within a year, the ailing King Richard died and the two ascended the throne of England. Within another year, the King of Scotland died creating a de facto union of the two lands (although legally the two remained separate nations sharing the same monarch and in perpetual alliance).
None of the royal couple's early children survived infancy. King James was on his deathbed at Falkland Palace on December 14 when his only living heir, a girl, was born. James Hamilton, 2nd Earl of Arran, the next in line for the throne, acted as Regent for the infant Queen Mary I, until 1554, when he was succeeded by the Margaret of York herself, until her own death in 1560.