What does “the little guy” in “(Larry Summers) would be an enthusiastic enforcer of bank regulation to protect the little guy” mean?

What does "the little guy" in "(Larry Summers) would be an enthusiastic
enforcer of bank regulation to protect the little guy" mean?

In New York Times' (August 13) article titled, "Summers of our
discontent," Maureen Dowd manifests her objection to the possible
nomination of Larry Summers by President Obama by asking, "Does the fact
that we've had no female Fed chairs and no female Treasury secretaries
mean that Summers was right when he said women are less likely to have the
kind of brains that would allow them to get top jobs requiring math
skills?
http://www.nytimes.com/2013/08/14/opinion/dowd-summers-of-our-discontent.html?hp&_r=0Summers
And she asks;
"But the idea that it is somehow historically inevitable that the
chairmanship of the Federal Reserve should go to Summers, that it belongs
to him, that he would be an enthusiastic enforcer of bank regulation to
protect the little guy? I have my doubts."
Who is "the little guy"? Does it mean loan borrowers? Is it a single
person or entity? Why is it in the singular form here?