The letter
Hamza does not originally belong to the original 28 Arabic letters. It is, however, derived from the Arabic letter Aein (ع). It is most often seen atop or below the letter.
But it is still considered one of the main vowels in the Arabic language.
If you want to know more about the Hamza and its forms, try to check on this article Learn Arabic Letters and Diacritics: Al-Hamza