Singular of "folks"?

There is no direct singular word for folks. It is almost like the word "guys" (but more gender neutral).

If an announcer comes out and says, "Howdy folks!" They are speaking to a group or crowd. You do not say folks when speaking to one person nor do you say folk. You would simply say "Howdy Mister/Madame/Whatever". Folks is usually a very friendly word that conveys a down to earth attitude. I think the closest word to takes its singular place would be friend.

If you see a group of people walking by that you know- "Howdy folks".

If you see a person walking by that you know - "Howdy friend" or just "Howdy". When you gave the "Howdy Folks" to the group walking by the use of folks noted that there was an individual howdy for everyone.