Rey Carlos V, also known as Charles V, was the Holy Roman Emperor from 1519 to 1556 and a pivotal figure in European politics during the Renaissance. His reign marked the height of Spanish power, including the establishment of a vast colonial empire in the Americas, which is crucial in understanding the context of Hernán Cortés's second letter, where he addressed his conquests and experiences in the New World, appealing for recognition and support from Carlos V.