Westmorland is one of Henry IV’s lords, the same character as in Henry IV, Part One.
Prince John sends him as an emissary to the rebels, with whom he argues at length over the justice of their cause. Mowbray’s misunderstanding of history leads him to scorn the young man. As the Prince’s second-in-command, he is an essential part of the plot to arrest the rebel leaders, convincing them that all is well, drinking with them, and offering a reasonable excuse for why the royal army has not dispersed. He has no philosophical difficulties with acting dishonorably towards traitors.