isolating shorty... but get an unexpected call

-I have played with villain quite a bit and he's alright, not fishy generally
-he runs at 18/12/2
-I was VERY confused by his overcall, no clue what he would be doing this with. Whatever it was, unless he was slowplaying, he didn't have the odds
-how bad is my flop check? I figured he must have some pair to call preflop, so he was on two outs and a check might get an extra bet on a later street. Turn bet again was a weak value bet, since I really didn't think he had much to call with here. Ax is possible I suppose.

Cryptologic
No Limit Holdem Ring game
Blinds: $1/$2
6 players
Converter

Stack sizes:
UTG: $277
UTG+1: $250.25
CO: $209.25
Button: $311.75
SB: $37
Hero: $429.75

Pre-flop: (6 players) Hero is BB with
UTG folds, UTG+1 calls, 2 folds, SB raises to $6, Hero raises to $37, UTG+1 calls, SB folds.

Flop: ($80, 2 players)
Hero checks, UTG+1 checks.

Turn: ($80, 2 players)
Hero bets $44, UTG+1 calls.

River: ($168, 2 players)
Hero bets $77, UTG+1 raises all-in $169.25, Hero ??

hero bets the flop

i think villain mis clicked limped (or called) pre flop, or just doesn't know set-odds.

either way i think you get more value from betting the flop.
KK will call once (probably), or else a set-hunter or mis-click is unlikely to put in $ unless they hit.