Rupert of the Rhine

Prince Rupert of the Rhine was a military commander, a nephew of King Charles I and cousin of King Charles II of England. He was fighting in English Civil War on the side of the Cavaliers.

He was dashing and handsome, with huge brown eyes, a thin, aquiline nose and a dimpled chin. (PROSE: The Roundheads)

On 26 July 1643, he met the Eleventh Doctor, Amy and Rory in Bristol. He used his poodle Boye to stun his enemies. (COMIC: Dog of War!)

Prince Rupert led an 18,000-man strong army that fought against Oliver Cromwell at Marston Moor in the English Civil War. He was soundly defeated. (PROSE: Nightshade)

In 1648, travelling under the alias Richard Godley, he managed to meet with the imprisoned Charles I. (PROSE: The Roundheads)

Behind the scenes
While Mark Gatiss did not use the title "of the Rhine" in The Roundheads, he stated that Charles I was Prince Rupert's uncle, thereby identifying him beyond any doubt.