Find the missing character?

The missing character is $\boxed{\text{S}}$.

The sequence is a list of letters, each of which is one letter after the previous letter in the alphabet. The first letter is $\text{A}$, the second letter is $\text{B}$, and so on. The missing letter is the letter that comes after $\text{U}$ in the alphabet, which is $\text{S}$.

A is the first letter of the alphabet, B is the second letter, and so on. U is the 20th letter of the alphabet, so the next letter in the sequence is the 21st letter, which is S.