I didn't see the guy's stats during the hand, but the previous hand he bet the flop multi-way with a gutshot with his AJ, and got it in on the turn with ace high against a bluff by a short stack. I thought that this way spewy play with a gutshot/ace high. He only showed down good starting hands, so he's like a spewy TAG as far as I could tell.

On the turn, I figured his range would be a lot of hands like AQ which I had beat, and KQ which beat me. Not so much AK because he doesn't benefit from raising there since he folds only hands he has beat.

On the river, my improving was a mixed blessing, since now the straight draw completed so if he was semi-bluffing on the turn he got there. However, if his turn raise was a value raise I got there. With his large river bet, it just doesn't seem plausible he would bet anything less than a set on a 4 straight board. But it's even more likely that he has air because it's a perfect board to bluff again.

Full Tilt No-Limit Hold'em, $0.25 BB (9 handed) - Full-Tilt Converter Tool from FlopTurnRiver.com

MP1 ($25.55)
MP2 ($200.05)
Hero (MP3) ($26.50)
CO ($32.40)
Button ($25)
SB ($10.35)
BB ($20.65)
UTG ($29.25)
UTG+1 ($4.65)

Preflop: Hero is MP3 with J, K
4 folds, Hero raises to $0.75, CO calls $0.75, 3 folds

Flop: ($1.85) 9, 10, K (2 players)
Hero bets $1.25, CO calls $1.25

Turn: ($4.35) 5 (2 players)
Hero bets $2.75, CO raises to $6, Hero calls $3.25

River: ($16.35) J (2 players)
Hero checks, CO bets $12, Hero?