Isabel de Portugal (24 October 1503 – 1 May 1539) was an Infanta of Portugal, by birth, and a Holy Roman Empress, Queen of Germany, Italy, Spain, Naples and Sicily, Duchess of Burgundy etc. as the spouse of Charles V, Holy Roman Emperor. She was the daughter of Manuel I of Portugal and Maria of Aragon. She served as regent of Spain during the absence of Charles V for long periods.
http://en.wikipedia.org/wiki/Isabella_of_Portugal accessed 23.04.14
She was much loved in Spain. Her presence in the vihuela repertoire is principally through the sonnets on her death set to music by Mudarra (mu059), and also in another song (mu067).Mudarra may well have known Isabel from when he was a page for the Dukes of the Infantado in his youth.