The missing character is D.
The first three characters are all vowels, and the last three characters are all consonants. The missing character is a consonant that comes between the vowels A and E. The only consonant that fits this description is D.
A is a vowel.
B is a consonant.
C is a consonant.
D is a consonant.
E is a vowel.
G is a consonant.
H is a consonant.
I is a vowel.
J is a consonant.
K is a consonant.
L is a consonant.
M is a consonant.
N is a consonant.
O is a vowel.
P is a consonant.
Q is a consonant.
R is a consonant.
S is a consonant.
T is a consonant.
U is a vowel.
V is a consonant.
W is a consonant.
X is a consonant.
Y is a consonant.
Z is a consonant.