2nd pair no kicker 5-handed

[Ampelmann]

I didn't have many hands with any player, but CO seemed to be a TAG (he definitely was tight).

PokerRoom 0.25/0.50 Hold'em (9 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

Preflop: Hero is BB with 3[img]/images/graemlins/club.gif[/img], 9[img]/images/graemlins/club.gif[/img].
UTG calls, <font color="#666666">1 fold</font>, MP1 calls, <font color="#666666">1 fold</font>, MP3 calls, CO calls, <font color="#666666">2 folds</font>, Hero checks.

Flop: (5.60 SB) 6[img]/images/graemlins/spade.gif[/img], K[img]/images/graemlins/heart.gif[/img], 9[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(5 players)</font>
Hero checks, UTG checks, MP1 checks, MP3 checks, <font color="#CC3333">CO bets</font>, <font color="#CC3333">Hero raises</font>

I thought that an aggressive player could bet many hands here. Since no one bet it's not impossible that there is no K around and a raise might force a better 9 out.

Thoughts?