$100NL WPX , full stacks

Villian seems like weak player but I have only played about 30 hands with him.
He called the turn bet pretty quick. And the river check was quick but not instant. I put him on a pocket pair but i dont think he would have KX here too often. Anybody value bet the river? Also, thoughts on the flop check?

** Dealing Down Cards **
Dealt to Hero (BTN):

UTG: Raise ($4)
Player 1: Fold
Player 2: Call ($4)
Hero (BTN): Raise ($20)
Player 4: Fold
Player 5: Fold
UTG: Call ($16)
Player 2: Fold

** Dealing Flop ** ($47)
Community cards:
UTG: Check ($0)
Hero (BTN): Check ($0)

** Dealing Turn ** ($47)
Community cards:
UTG: Check ($0)
Hero (BTN): Bet ($33)
UTG: Call ($33)

** Dealing River ** ($113)
Community cards:
UTG ($47 left): Check
Hero (BTN): ???

I never bet this.