He was the youngest son of Juan de Jasso, privy councillor to Jean d'Albret, king of Navarre, and his wife, Maria de Azpilcueta y Xavier, sole heiress of two noble Navarrese families.
He won over the Savoyard Pierre Lefevre (Faber), whose room he shared, and the Navarrese Francis Xavier, who taught philosophy in the college of St Barbara.
In May 1364 he won an important victory over the Navarrese at Cocherel, and took the famous Captal de Buch prisoner.
At the same time he endeavoured to bring about a union of Aragon with Navarre, by a contract of mutual adoption between himself and the Navarrese king, Sancho, who was old enough to be his grandfather.
In the middle ages the name Pylos was replaced by that of Avarino ('A(3apvos) or Navarino, derived from a body of Avars who settled there; the current derivation from the Navarrese Company, who entered Greece in 1381 and built a castle at this spot, cannot now be maintained (Eng.