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.}

There's no entry in the OED for little guy but there is for poor little guy:

U.S. colloq. the ordinary individual, the ‘man in the street’.

First uses:

[1863 C. Reade Hard Cash III. xiii. 270, I wouldn't speak to you in the street for fear of disgracing you; I am such a poor little guy to be addressing a gentleman like you.]

1955 Bridgeport (Connecticut) Sunday Post 14 Aug. a16/7 By loading down the audit forces with detail work and the package audit, less audits will be made, and then mostly to the poor little guy.

However, they do have an entry for little man:

The undistinguished and ordinary ‘man in the street’.

1933 E. Sutton tr. H. Fallada (title) Little man, what now?

1935 New Statesman 8 June 857/1 The old noli-me-tangere John Bull has disappeared, and his place has been taken by the all-enduring Little Man.

1936 ‘G. Orwell’ Keep Aspidistra Flying iii. 64 To turn into the typical little bowler-hatted sneak—Strube's ‘little man’.