The missing character is $\boxed{\text{K}}$.
The other three characters, $\text{A}$, $\text{E}$, and $\text{R}$, are all vowels. Vowels are letters that make a sound when you say them, and they are used to make words. The letter $\text{K}$ is a consonant. Consonants are letters that make a sound when you say them, but they are not used to make words by themselves.
In the question, the four characters are arranged in a circle. The first character is $\text{A}$, the second character is $\text{E}$, the third character is $\text{R}$, and the fourth character is $\text{K}$. The fifth character is the first character, and so on.
The missing character is the consonant that comes after $\text{R}$ in the alphabet. The consonant that comes after $\text{R}$ in the alphabet is $\text{K}$. Therefore, the missing character is $\text{K}$.