The missing character is Q.
The other letters in the sequence are all consonants that are pronounced with a voiceless velar plosive sound, which is the sound made by placing the back of the tongue against the soft palate and then releasing it suddenly. The letter Q is also pronounced with a voiceless velar plosive sound, so it fits the pattern of the sequence.
The letter A is a vowel, so it does not fit the pattern of the sequence.
The letter H is a voiceless glottal fricative, which is the sound made by constricting the vocal cords without vibrating them. This sound is different from the voiceless velar plosive sound, so the letter H does not fit the pattern of the sequence.
The letter P is a voiceless bilabial stop, which is the sound made by placing the lips together and then releasing them suddenly. This sound is different from the voiceless velar plosive sound, so the letter P does not fit the pattern of the sequence.
The letter G is a voiced velar plosive, which is the sound made by placing the back of the tongue against the soft palate and then releasing it suddenly. This sound is similar to the voiceless velar plosive sound, but it is pronounced with the vocal cords vibrating. This difference is not enough to make the letter G fit the pattern of the sequence, so it is also not the correct answer.