Ferdinand I of Aragon

Born in November 1380, Ferdinand was a Castilian royal who spent a decade as regent of Castile starting in 1406, steering the kingdom through a minority reign. In 1412 he broke the mold entirely: elected king of Aragon, he became the first Castilian to hold that crown, inheriting Valencia, Majorca, Sardinia, Sicily, and a string of Mediterranean titles from Barcelona to nominal Athens. He ruled both realms simultaneously until his death in Igualada on 2 April 1416, four years into the Aragonese throne. The epithet "the Just" stuck.