Y, It’s All Greek to Me
Simple answer - it’s a vowel. Or - at least - it started out that way. But do you want to know why? Blame the Greeks.
And so - the slightly longer answer…
The Greek alphabet has 24 letters, seven of which are vowels. These seven are:
α (alpha) - the equivalent of a in the Roman alphabet
ε (epsilon) - the equivalent of ‘short’ e
η (eta) - the equivalent of ‘long’ e
ι (iota) - the equivalent of i
ο (omicron) - the equivalent of ‘short’ o
υ (upsilon) - the equivalent of u
ω (omega) - the equivalent of ‘long’ o
Like the Roman alphabet, the Greek alphabet has a set of capital letters that complements the small letters. In the case of the vowels, the capital and small letter pairings in Greek look like this:
Αα Eε Hη Iι Oο Yυ Ωω
In some cases (A, E, I and O), the Greek capital letter look the same as their Roman counterpart. Capital eta, confusingly, looks like a capital H. Capital omega looks quite unlike any Roman letter. Which leaves us with capital upsilon. And that looks just like a Roman capital Y (I say ‘Roman capital Y’, despite the fact that Classical Latin didn’t actually have a letter Y at all. It was ‘imported’ into English - which otherwise generally used the Roman alphabet - from Greek. Precisely the point I’m making here, of course. Blame the Greeks.)
And that is also why certain words that are spelt with an upsilon in Greek are spelt with a ‘y’ in English, where the English words are derived directly from the Greek. Let me give you a few examples. The English word ‘psyche’ (which also gives us other similar words, like ‘psychologist’, psychiatry’ and ‘psycho’), is derived from the Greek word ψυχη (psuche) - notice that the second letter is upsilon. Another example is ‘hypnosis’, an English world derived from the Greek word υπνωση (upnose). Here, the first Greek letter, an upsilon, has ‘turned’ into a ‘y’ in English. One final example: the English word ‘synagogue’ is derived from a not-unexpectedly common word that is found within the Greek New Testament, συναγωγη (sunagoge) - once again, note how a Greek upsilon has been rendered with a ‘y’ in English (and in all these examples, is actually pronounced ‘i’, whether long or short, and not ‘u’).
So, the Greek vowel upsilon is, effectively, the ancestor of two letters in English - U and Y. And ‘y’, therefore - at least in terms of origin - is to be considered a vowel, not a consonant.
Of course, over time it acquired a usage as a consonant too. But that - as they say - is another story.
Then, of course, there’s the way in which ‘i’ in Latin could be both vowel and consonant - and when pronounced as a consonant was pronounced as a ‘y’ (and, eventually, rendered as a ‘j’). So, for example, Iove (pronounced ‘Yove’) eventually gives us Jove, Iupiter (pronounced ‘Yupiter’) eventually yields Jupiter, and Iulius… well, you get the picture. But that’s yet another story.
There are also the occasions when ‘y’ in English actually derives from an archaic Anglo-Saxon letter for ‘th’ - which is why in the phrase ye olde tea shoppe, the ‘ye’ should actually be pronounced ‘the’ (contrary to what most people assume). But that is yet another, entirely different, story!
And then there’s ‘y’ in Welsh...
I’m going to stop now. You did want the simple answer - right?