The king of hearts is the only king without a mustache

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. The most common names for the kings were: 

* King of Spades: David (a biblical king)
* King of Hearts: Charles (presumably after Charlemagne)
* King of Diamonds: Caesar (Julius Caesar)
* King of Clubs: (Alexander the Great)