Flop just a standard CB, but on turn, I decided to slow down since I could easily be behind and didnt want the pot to get any bigger. After the flop, he has $25 left.
Who checks turn here?

PokerStars No-Limit Hold'em, $0.50 BB (6 handed) from (Format: Bet The Pot)

Button ($24.40)
SB ($19.50)
Hero ($50)
UTG ($34.25)
MP ($24.30)
CO ($41.75)

Preflop: Hero is BB with , .
UTG calls $0.50, 2 folds, Button calls $0.50, SB completes, Hero raises to $4, UTG calls $3.50, Button folds, SB folds.

Flop: ($9) , , (2 players)
Hero bets $6, UTG calls $6.

Turn: ($21) (2 players)
Hero checks, UTG checks.

River: ($21) (2 players)
Hero?

Is he a nit, or a donk or what? I would just CB the turn, right around 15 bucks - especially if he is a donk.