María de las Mercedes, Princess of Asturias (María de las Mercedes de Borbón y Habsburgo-Lorena) (11 September 1880 – 17 October 1904), was the eldest child of King Alfonso XII of Spain and his second wife, Maria Christina of Austria. She was Princess of Asturias, the heir presumptive to the Crown of Spain, for all 24 years of her life.

Had her younger sibling, unborn at the death of Alfonso XII, been a daughter, Mercedes would have been queen regnant of Spain. The sibling proved to be a boy, Alfonso XIII, and on his birth in 1886, Mercedes turned out not to be queen. She resumed the position of heir presumptive, which she held until her own death, and was succeeded in it by her own infant son Alfonso, Alfonso XIII having not yet married and fathered a legitimate child.

Mercedes married in Madrid on 14 February 1901, her second cousin, Prince Carlos of Bourbon-Two Sicilies, a nephew of the King of the then-defunct Kingdom of the Two Sicilies, who was elevated to the rank of Infante of Spain. The marriage was highly controversial due to her father-in-law's ties with the Carlists. She died three years later from complication while giving birth to her third child.

Born on 11 September 1880 at the Royal Palace of Madrid, Mercedes was the eldest daughter of King Alfonso XII and his second wife, Maria Christina of Austria. She was christened María de las Mercedes Isabel Teresa Cristina Alfonsa. Her godmother was Queen Isabella II, her paternal grandmother, who came from retirement in Paris to attend the birth of her first grandchild. There was great disappointment because family and nation were hoping for a boy. To smooth thing out, Queen Maria Christina suggested giving her daughter the name Mercedes in honor of her husband's first wife, Mercedes of Orléans.

...
Wikipedia