Edmund is the younger and illegitimate son of the Earl of Gloucester. He resents being treated differently from his older, legitimate half-brother, Edgar. He secretly plots against his father and brother to gain their lands and title.

He impresses the Duke of Cornwall when he shows his father up as a traitor for secretly receiving letters about the French invasion. Cornwall rewards Edmund by making him Duke of Gloucester instead of his father. Edmund swears his love to both Goneril and Regan and when Cornwall dies, Regan puts him in charge of her troops and intends to marry him.

After the battle against the French, because of all his betrayals, Edmund is challenged to a duel by Edgar, who kills him.