Find the missing character?

The missing character is X.

The other three characters, A, S, and M, are all letters that appear in the alphabet twice. X is the only letter that appears only once in the alphabet.

Here is a table showing the number of times each letter appears in the alphabet:

Letter | Number of appearances
——- | ——–
A | 2
B | 1
C | 2
D | 2
E | 3
F | 1
G | 2
H | 1
I | 3
J | 1
K | 1
L | 2
M | 2
N | 2
O | 2
P | 1
Q | 1
R | 2
S | 2
T | 2
U | 2
V | 1
W | 1
X | 1
Y | 2
Z | 1