The figure is habited in grotesquely ragged and ill-assorted garments (whence sense 2), and was formerly accompanied by other similar effigies (representing unpopular persons), to which the name of ‘guys’ is often given by extension.
2. That in turn produced “guy” in the sense of “A person of grotesque appearance, [especially] with reference to dress; a ‘fright.’”
3. And this in turn developed in the U.S. into just “A man, fellow”; the Oxford English Dictionary notes that “The earliest examples may be influenced by sense 2,” which is to say “person of grotesque appearance” meaning, which in turn flows from Guy Fawkes.
The name Guy, it turns out, is the Norman French analog of the Italian name Guido. Apparently Guy Fawkes himself went by Guido Fawkes, back in the day.
So, you guys, you rebels you, that’s where your name comes from. Probably.