The Hawaiian alphabet has only 12 letters.

The Hawaiian language was first developed in the 1820s by Protestant missionaries from New England. Prior to contact from the Western world, Hawaiians had no written language other than petroglyph symbols.

In order to convert all the Hawaiians to Christianity, they needed to standardize the native language and teach the locals how to read and write it. This goal was accomplished in 1826 in addition to the publication of a Hawaiian Bible.

The modern Hawaiian alphabet contains five vowels, two consonants, and a symbol called an 'okina (') which marks a phonetic glottal stop (it is also technically considered to be a consonant). Hawaiian words only end in vowels, and each consonant must also be followed by a vowel.

The twelve letters that are included in the Hawaiian alphabet are as follows: A, E, I, O, U, H, K, L, M, N, P, and W.
(source)