Daughter of the Red King, Arianna has been alive since prehistoric times. She has hated Europeans since Conquistador Hernán Cortés' exploration of Mexico, the Caribbean and Central America (early 16th century) and his subsequent conquest of the Aztecs.
In Changes she goes to the White Council's HQ in Edinburg as an ambassador plenipotentiary. Her mission to negotiate peace is a ruse ment to further her plot of revenge against Harry, lull the White Council into a false sense of security while the Red Court are massing for a new attack and infect the White Council with a disease. She is abetted in this by new Senior Council member Cristos.
In Changes, Arianna Ortega found out about Margaret Angelica, daughter of Harry Dresden, and had her kidnapped, to use her in a bloodline curse to wipe out her family. Arianna was later killed by Harry. Her plan to kill Dresden's family not only failed, but led to the destruction of the Red Court.