A knight who fell at Roncesvalles, cousin to Montesinos. The tale says he loved Belerma, whom he served seven years, at the expiration of which time he was slain. In his last breath he told Montesinos to take his heart and give it to Belerma. He is described by Lewis as
Sweet in manners, fair in favour, Mild in temper, fierce in fight.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Durandarte from Infoplease: