The missing character is M.
The sequence is a simple substitution cipher, where each letter is replaced by the letter three places after it in the alphabet. So, A becomes D, B becomes E, and so on. The missing character is the third letter after M, which is P.
Here is a table showing the substitution cipher:
Letter | Replaced by ------- | -------- A | D B | E C | F D | G E | H F | I G | J H | K I | L J | M K | N L | O M | P N | Q O | R P | S Q | T R | U S | V T | W U | X V | Y W | Z Z | A
I hope this explanation is helpful!