A muse, a bard or a storyteller, to give a no-holds-barred account of such tales as Apollo and Hyacinthus, Zeus and Ganymede, Iphis and Ianthe - but not just Greek, there's a whole bunch of cool stuff the Norse gods got up to that an irreverent commentator could tell, and then there's a load of obscure Russian stories and so on and so on.
Anyway, my point is that my commentator should not be particularly scared of the gods, and should not be overly pigeonholed - this last is why one of the Muses won't do, although I am considering Mnemosyne. Should also preferably be female, although this isn't essential. Greek is not necessary.
Have Googled muses, god of memory, memory deity, bard deity (this last turned up Bridgid, of the Celts, which I may use if there's nothing else - I'm not that up on Celtic mythology, and really I'm looking for something with a little more punch).
Have also been through some of Encyclopedia Mythica, but it's big and hard to navigate if you're looking for deity types!
Edit - Thanks people, you've all been very helpful! I'm gonna use either Orpheus or Ratatosk, I think - gonna go think on this...