William Marshal

Shield with the arms of William Marshal, British Library
William Marshal (c. 1146–1219), 4th Earl of Pembroke, was served four English kings. He was among King John’s loyal supporters and was one of the chief mediators in the years leading up to the granting of Magna Carta.
Following King John’s sudden death in 1216, William Marshal was appointed regent in place of Henry III, who was only nine years old. William was responsible for issuing revised versions of Magna Carta in 1216 and 1217, which secured baronial support for the young king. He died in 1219 and was buried in the Temple, London.
Further information about the life of William Marshal can be found via the Oxford Dictionary of National Biography.