In the standard English playing card deck, the king and the other face cards represent no one in particular (though the king of hearts is sometimes referred to as the “suicide king” because he appears to be sticking his sword into his head; evidently, the king of hearts is also the only one of the kings without a moustache, whereas the king of diamonds is the only king not be depicted carrying a sword, wielding an axe instead).
However, the court cards in a French deck do have names. Because the manufacture of playing cards was illegal in the UK during the Interregnum, when the English Restoration came and the court began playing card games, the suits in an English deck came from the French deck, but without all of the lore.
For a period, starting in the 15th century, French playing-card manufacturers assigned to each of the court cards names taken from history or mythology. This practice had largely disappeared by the 19th century. The most common names for the kings were:
The king of diamonds is the Roman statesman and general Julius Caesar; the king of clubs, the Macedonian conqueror Alexander the Great; the king of spades, the biblical King David; and the king of hearts, the Frankish King Charlemagne.