top pair vs. unknown

SB is on his second hand at the table
button is fairly laggy

Paradise Poker 8/16 10max, 5handed

Preflop: Hero is CO with K[img]/images/graemlins/heart.gif[/img]9[img]/images/graemlins/club.gif[/img].
1 fold, Hero raises, Button calls, SB calls, BB calls.

Flop: 4[img]/images/graemlins/club.gif[/img] 8[img]/images/graemlins/diamond.gif[/img] 9[img]/images/graemlins/spade.gif[/img]
SB checks, BB checks, Hero bets, Button calls, SB raises, BB folds, Hero raises, Button folds, SB caps, Hero calls.

Turn: 2[img]/images/graemlins/diamond.gif[/img]
SB bets, Hero calls.

River: A[img]/images/graemlins/spade.gif[/img]
SB bets, Hero calls.

standard or should i fold somewhere?