I had just assumed he was the younger version of Uriel's original vessel. As far as I know there are speculations that some are vessels which "fit" some angels better than others. The "weaker" the angel the more possible vessels, but there is still one which fits the best.
"The Soul-Piece" recently had a chapter about the choosing of vessels...

I'm actually quite disappointed that it was Micheal who killed Anna. I wouldn't have minded so much if it had been Castiel who'd killed her because I imagine he would have been really conflicted over it afterwards and it would have showed off his loyalty to the Winchesters again. (More so than killing those random angels because he and Anna seem to have shared some sort of bond).
And if he'd saved them the whole "angels are watching over you" would have made more sense... *sigh* It's pretty creepy that Micheal just manipulated Mary like that though... *shivers*

BTW, what did you think of Micheal? I loved how John's actor pulled it off (scaaary) but I would have liked it better if they had done something else with Micheal's character...

IN CONCLUSION: More Cas. And I kind of liked the episode *drop*