Blind defense check up

[NinaWilliams]

Villain is unknown. Whats the plan if villain bets river?

Full Tilt Poker
Limit Holdem Ring game
Limit: $5/$10
4 players
Converter

Pre-flop: (4 players) Hero is BB with A[img]/images/graemlins/heart.gif[/img] 8[img]/images/graemlins/diamond.gif[/img]
UTG folds, <font color="#cc0000">Button raises</font>, SB folds, Hero calls.

Flop: K[img]/images/graemlins/heart.gif[/img] 2[img]/images/graemlins/diamond.gif[/img] 7[img]/images/graemlins/heart.gif[/img] (4.5SB, 2 players)
Hero checks, <font color="#cc0000">Button bets</font>, Hero calls.

Turn: 5[img]/images/graemlins/diamond.gif[/img] (3.25BB, 2 players)
Hero checks, <font color="#cc0000">Button bets</font>, Hero calls.

River: 5[img]/images/graemlins/spade.gif[/img] (5.25BB, 2 players)
Hero checks,