Most of the time it is not used as a vowel. It is only a vowel in words such as 'why', but in words such as 'yes' it is not. An acceptable solution to the Kata allows the letter y to be classified as a vowel in both cases, which is incorrect. In order for the Kata to be correct, it needs to be reclassified as being significantly harder as it would need to consider the structure of words for common patterns of where y is a vowel and not a consonant. Or, remove yas a vowel and stick with the commonly agreed a, e, i, o and u.