The missing character is R.
The first three characters, A, B, and C, are all letters of the alphabet. The fourth character, I, is not a letter of the alphabet. The fifth character, R, is a letter of the alphabet.
The first three characters, A, B, and C, are all in the first row of the alphabet. The fourth character, I, is in the third row of the alphabet. The fifth character, R, is in the first row of the alphabet.
The first three characters, A, B, and C, are all consonants. The fourth character, I, is a vowel. The fifth character, R, is a consonant.
The first three characters, A, B, and C, are all in the first half of the alphabet. The fourth character, I, is in the second half of the alphabet. The fifth character, R, is in the first half of the alphabet.
The first three characters, A, B, and C, are all in the first quarter of the alphabet. The fourth character, I, is in the third quarter of the alphabet. The fifth character, R, is in the first quarter of the alphabet.