Short answers, pulled from the story.
Oliver Cromwell became Lord Protector of England Scotland and Ireland on the 16th of December 1653. He accepted the title before the House of Commons and transformed the nation from a republic into a military dictatorship disguised as a commonwealth. This event marked the first time the title was used in English history.
John of Lancaster Duke of Bedford and Humphrey Duke of Gloucester jointly served as Lords Protector for the infant Henry VI. They governed the kingdom from the 5th of December 1422 until the 6th of November 1429. These men were regents tasked with defending the realm while the young heir grew into his role.
Richard Cromwell assumed the title on the 3rd of September 1658 and his reign lasted only eight months. He resigned on the 25th of May 1659 unable to command the loyalty of the army or the support of parliament. The failure of the Protectorate led to the restoration of the Stuart monarchy under Charles II in May 1660.
In medieval times the term described a temporal lord who acted as the protector of the secular interests of a part of the church. The Lord Protector was expected to defend the church from external threats while also ensuring that the church did not overstep its bounds in political matters. This dual function created a unique balance of power essential to the stability of the kingdom.
Mary Shelley's novel The Last Man features a republic with its elected head of state styled as Lord Protector. The 2008 film adaptation of C. S. Lewis' Prince Caspian featured the antagonist Miraz who began as Lord Protector of Narnia before being proclaimed King. The 2020 film Wolfwalkers also features the Lord Protector as the primary antagonist representing oppressive forces.