can villain have anything but KK here? or is folding weak?

Full Tilt No-Limit Hold'em, $0.50 BB (9 handed) Full-Tilt Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

MP3 ($30.85)
CO ($8.20)
Button ($28.50)
Hero ($48)
BB ($29.50)
UTG ($107.80)
UTG+1 ($51.70)
MP1 ($30.85)
MP2 ($30)

Preflop: Hero is SB with 8, 8.
UTG raises to $2.25, 3 folds, MP3 calls $2.25, 2 folds, Hero calls $2, 1 fold.

Flop: ($7.25) 8, K, 3 (3 players)
Hero checks, UTG checks, MP3 checks.

Turn: ($7.25) 4 (3 players)
Hero bets $5, UTG raises to $13.5, MP3 folds, Hero raises to $27, UTG raises to $83.5, Hero ???

Final Pot: $108.75