Mary II
Mary II was the queen and co-monarch of England, Scotland, and Ireland alongside her husband William.
History
Mary was the eldest daughter of James II. Because most of her siblings died young, she spent a significant portion of her childhood as second-in-line for the British crown. Furthermore, her father's rights to succeed were challenged through the Exclusion Crisis.
Mary was engaged to her cousin William, who was himself fourth-in-line, amid the Third Anglo-Dutch War. This marriage significantly improved her position as an alternate claimant to the throne.
Reign
James did succeed the English throne in 1685, but his reign was immediately plagued by unrest. Furthermore, it was believed that James would side with France in the rapidly-escalating conflict on the continent. William landed an army in Devon in November 1688. After failing to rally his own army, James fled to France. This Glorious Revolution crowned William and Mary as co-monarchs over England, Scotland, and Ireland.
William spent most of their reign abroad, commanding armies. The Nine Years War quickly erupted as expected. Then the Williamite War emerged as a theater of that conflict, through a Jacobite rebellion in Ireland.
Mary died in 1694, leaving Anne as the only feasible heir.