€2 NLHE 6-max: Small river bet - give villain credit for flush?

Microgaming - €0.02 NL (6 max) - Holdem - 6 players
Hand converted by PokerTracker 4

MP: 218.5 BB (VPIP: 23.91, PFR: 22.28, 3Bet Preflop: 9.23, hands: 199)
CO: 105 BB (VPIP: 26.42, PFR: 20.75, 3Bet Preflop: 0.00, Hands: 66)
Hero (BTN): 100 BB
SB: 106 BB (VPIP: 24.47, PFR: 14.89, 3Bet Preflop: 5.71, Hands: 95)
BB: 97.5 BB (VPIP: 0.00, PFR: 0.00, 3Bet Preflop: -, Hands: 3)
UTG: 100 BB (VPIP: 0.00, PFR: 0.00, 3Bet Preflop: 0.00, Hands: 5)

SB posts SB 0.5 BB, BB posts BB 1 BB

Pre Flop: (pot: 1.5 BB) Hero has K:club: A:club:

fold, fold, CO raises to 3 BB, Hero raises to 9 BB, fold, fold, CO calls 6 BB

Flop: (19.5 BB, 2 players) T:heart: 6:club: 9:heart:
CO checks, Hero bets 11 BB, CO calls 11 BB

Turn: (41.5 BB, 2 players) K:diamond:
CO checks, Hero bets 22 BB, CO calls 22 BB

River: (85.5 BB, 2 players) 5:heart:
CO bets 21 BB, Hero ???

What do you think? Could I have extracted more on flop?
Also, does his range include enough Kx and other hands for 1/4 pot call to be acceptable? I know this donkbet is a clear indication he has it most of the time, but how often?

Awfully wet connected board for villain to be bluffing at, I'd have to fold here.

Ummm... Insta call? It's more of a blocking bet by a T than a giant hand. If he made a flush in a 3-bet pot 100 deep, pay the guy.

No screaming t here..

No one bets in here light .. we bet every street with k..

Its a fold