http://en.wikipedia.org/wiki/X-Men
I started it off on the X-Men page, but you can easily find almost every character in marvel, and it goes VERY deep into detail about everything pertinent to each character…so be prepared to read ALOT. Personally I love marvel comics and the complexity of the Marvel universe. If you are interested in Marvel at all I would defiantly check this stuff out. its insane how much info is on there.
