Both Villain and hero are not HU regs. Played HU as table was breaking.

Villain is quite fishy and face up; has check raised the flop 2 times vs me in the past hour when the game was still 6 handed. Both times he showed top two pair. On one hand, he jammed 100BB with aces over a 3x and somehow got called by K6 and held lol.

Onto the hand:

Pre - Hero 3x on BTN with K5, Villain flats.

Flop ($12) 4 10 8 Hero cbets $6, V calls.

TURN ($24) - 6 Hero picks up a flush draw, V donks $12 and hero flats.

RIVER ($48) - K V checks, hero bets 22 and V raises to 72.

Hero

vs anyone with bluffraising range on this river, this is an easy call. However, fish like the one described tend to have no x/bluffraise range on the river, so it is likely just as easy fold.

btw river is not a thin valuebet, you are near the top of your range, you can never check this

I'm probably not calling the river c/r even tho it's a suspicious line from villain. Does he suspect you'll always bet behind every time he misses a cbet?
The donk ott and the raise otr indicate 75 or 97, but his 1/2 pot ott and the check otr indicate weakness. Weird. He might also think your underbet otr is weak and be putting u on a middle pair.
You have to give a higher than normal bluffing % given the strange line, but idk if it's enough to call especially given that you called him "face up." Vs a maniac/perpetual bluffer, I'm calling.

What does it mean when someone donks this turn? Its not a turn that is noticeably better for their range than yours. Both of you can have 97 and 57 is in your range at least as much as his. So there are really 3 possibilities:

1. He's just clicking buttons. Fish doing fish things isn't unheard of. Unless you have a ton of reads you can't be sure that, if you saw his hand, there would be any logic to this move at all. This is less common but not impossible.

2. He picked up equity. Perhaps he has Ax of spades or QJ of spades or A9 or something.

3. He made his hand with T6, 66 or one of the straights.

Before I started writing this I was thinking "well he probably either picked up a fd or made a hand stronger than 1 pair and we block fds so that means fold". Thinking through specific combos though you really only block A5s as a hand that might be an excellent candidate to play this way as a bluff so its not that relevant.

I lean towards fold without reads and since your observation that he had strong hands when check raising before doesn't contradict that I prefer fold. I would prefer to bet more on the river though. Its better against his bluffcatchers and I would be more confident folding after a bet of 35 or so. Then we don't have to worry as much that we induced him.