To the OP, you don't need to be able to store "either a letter or a number" if you are just trying to store the value of a playing card. A playing card consists of two items: a value and a suit.
The value is easily represented as an integer 1...13 and a suit is easily represented as an integer 1..4. The card "king of spades" is just another way to say I mean the ordered pair (13,4).
A simple way to represent an ordered pair in C is with two variables x and y. A more sophisticated way is to use a struct and reference the items myCard.value and myCard.suit for example.