The Earl of Northumberland is the same as the Northumberland of Richard II and Henry IV, Part One.
Having once helped Henry IV to usurp the throne, he is now theoretically rebelling against him. However, at the beginning of the play he is still pretending to be sick, which was his excuse for not joining his son Hotspur at the battle of Shrewsbury, leaving the latter under-strength; and he will later choose not to reinforce the Archbishop of York and the other rebels, with much the same results. The death of his son breaks his heart and leads him to rage against the King and help organize the second rebellion, but he is convinced by his wife and daughter-in-law to wait and see who will overcome rather than join in from the start. Waiting in Scotland, he raises an army with Lord Bardolph, but is beaten in battle by the Sheriff of Yorkshire. He is an old man, highly respected and powerful.