lines like these always confuse me. It seems a set/draw and the latter all got there. Is this one of those thin value bets?

villain is 56/20/2 over 87 hands.

Bet of check behind, and why?

PokerStars No-Limit Hold'em, $0.50 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

saw flop|saw showdown

Hero ($51.50)
MP ($23.10)
CO ($26.10)
Button ($49.05)
SB ($65.75)
BB ($68.35)

Preflop: Hero is UTG with Q, K.
Hero raises to $2, 3 folds, SB calls $1.75, 1 fold.

Flop: ($4.50) Q, 7, 9 (2 players)
SB checks, Hero bets $3, SB raises to $6, Hero calls $3.

Turn: ($16.50) 5 (2 players)
SB checks, Hero bets $9.5, SB calls $9.50.

River: ($35.50) J (2 players)
SB checks, Hero???

Final Pot: $35.50