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

The big guy and the little guy are metaphorical ways of referring to the distinction between corporations or individuals with lots of political and/or economic power and individuals with little or no political and/or economic power. So yes, in this case it means the weak, or more precisely, people in the middle or lower classes that have less individual power—people that are more personally influenced by local economic conditions.

Neither the big guy nor the little guy is a single entity. Both are collective terms describing entire economic classes.

Grammatically, the terms are indeed countable. The plural forms, the big guys and the little guys, can be used almost interchangeably with their singular forms, although they are much less common.

_{On a side note, I had speculated that the big guy / little guy distinction might have arisen from Big Brother, a character in George Orwell's novel Nineteen Eighty-Four, but it's been pointed out (see comments) that these terms predate the novel by at least several years.}