Final answer:
The first king of Spain was King Ferdinand II of Aragon, who united the kingdoms of Aragon and Castile. He played a key role in completing the Reconquista and establishing Spain as an imperial power.
Step-by-step explanation:
The first king of Spain was King Ferdinand II of Aragon. He married Queen Isabella of Castile in 1469, which united the two kingdoms and laid the foundation for modern Spain. Ferdinand and Isabella completed the Reconquista, the Christian conquest of the Iberian Peninsula, in 1492, which marked a significant step forward in establishing Spain as an imperial power.