The missing character is B.
The sequence is a simple alternating repetition of the letters A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, and Z.
The first letter is A, and the second letter is B. The third letter is C, and the fourth letter is D. The fifth letter is E, and the sixth letter is F. The seventh letter is G, and the eighth letter is H. The ninth letter is I, and the tenth letter is J.
The eleventh letter is K, and the twelfth letter is L. The thirteenth letter is M, and the fourteenth letter is N. The fifteenth letter is O, and the sixteenth letter is P. The seventeenth letter is Q, and the eighteenth letter is R. The nineteenth letter is S, and the twentieth letter is T. The twenty-first letter is U, and the twenty-second letter is V. The twenty-third letter is W, and the twenty-fourth letter is X. The twenty-fifth letter is Y, and the twenty-sixth letter is Z.
The missing letter is B, which is the third letter in the sequence.