I have only played 3 hands with villain, and I don't remember exactly what he did, but he played them donkish. I don't remember every street and if he was aggressive or passive or what.

Full Tilt No-Limit Hold'em, $1 BB (6 handed) Converter Tool from (Format: Bet The Pot)

Button ($102.50)
SB ($153.80)
BB ($133.85)
UTG ($100)
Hero ($100)
CO ($55.20)

Preflop: Hero is MP with , .
1 fold, Hero raises to $3.5, CO calls $3.50, Button calls $3.50, 2 folds.

Flop: ($12) , , (3 players)
Hero bets $10, CO calls $10, Button folds.

Turn: ($32) (2 players)
Hero?

If I check and he bets $21, what do I do?

You fold...

check/fold