Moved to main game, had 1,900, I'm not from this area but probably viewed as good aggresive player by villian based on table presence, my hood up and I had been at table only 20 minutes had been already won a few hands without showdown, also straddled button ect.

Button straddles, mid 20s nerdy looking kid completes BB I raise 10h10s 50 utg button calls he calls. 6c5c2h he checks I bet 80 button folds he calls turn Jx he checks I bet 175 he calls. River 2s he checks I bet 250, obviously thin but with my image and the fact that I just have the best hand here so much I go for it, obviously I value own myself vs Jxcc here some times but I'm targeting 77-99 and 33/44 or 6x or 5x that just doesnt wanna beleive me. He goes all in verbally for 815 more. I tank for 2 minutes, he offers to show me a card, he flashes a Kd... Hero?
Only extra info I knew is my friend told me he had double floated him before I got to the main. I didn't know the details of the hand though.

I like the way you played all the way till the river.
Him showing the Kd is really weird, obviously weakens his range a great deal, is he really trying to sell us KK or K2dd? I can't imagine him having KJcc/hh here as it's the simplest call for him on the river. Makes no sense for him to show this card unless he's got you beat, did it seem like you were about to fold or to call? Did it seem like it was a desperate attempt to get you to change your action?
Bottom line is I can't imagine him ever having KK, K2 or KJ, so I just shrug call.

I had counted out the bet and moved in a little closer to the line separate from my stack. It looked to him like I really wanted to call I'm sure.
Also you don't like the thin value bet on the river you mean?

I'd fold , he's never showing a card like that unless he has you beat

SB range with Mississippi straddle is insanely tight and can absolutely include KK as a limp call imo. Also it doesn't make sense for him to get thru flop and turn with KJ or K2 either, it can only be KK or AK

I like the vbet and the sizing. If your intent to call was clear, him showing would make me call even more.
I disagree we get shown KK ever here, he has to limp/rr it at least 50% of the time preflop, he is then definitely c/raising flop or turn with the remaining 3 combos, and thus has no combos left after this action on the river (we can maybe count 0,5 combo).
What I didn't add to my original post was that his most likely hand to do this with is AK of which he can definitely have a pretty good amount of combos (say 6 out of 18), which more than compensates the times we lose to his stronger holdings.
He might also have turned over the King thinking it was the Ace, which would make more sense as a bluff.

Him showing you the Kd seems to be a desperate attempt to make you fold. So call.

I refuse to pay off anyone who check raises the river against me at 2-5 or below. I just don't think enough of them are capable of bluffing in those spots. I fold (Hint: He doesn't have KJ)

Check river ldo

Pretty big live tell when a fish/bad player does anything with their cards in response to you counting out chips for a call. Especially if you had been tanking for awhile before this happened.

If you think villain isn't terrible (hard to believe considering he is flashing the Kd and the way he played the hand so far) he's got some weird badly played KK most of the time, but then again if he ever has KK here that means he's probably a fish so we go right back to the above live read and call.

Not moving the chips (i.e. verbal shove) mostly means he know he's getting it back, weird card showing and being comfortable on the river for what you describe as a nerdy 20's kid, also means he knows he getting it back. From a range perspective, you have the bottom of your value betting range on the river and although the Kd makes little sense, 2 buy in shoves on the river without further information mean (in their minds) they have the nuts.

Fold

C/R from this type of player is rarely ever a bluff.

Real Question is: Would he show any cards if he was bluffing?

IMO, shouldn't have bet the river.

As played, I'd be pretty indifferent to calling/folding. Showing K means he's got 3 combos that beat you... KJ, KK, and K2. Given the draws out there, I think he'd check raise KK at some point. KJ is a bit light of a shove. This is probably K2.

What did you look like you were probably going to do while tanking before he showed you the card?

Got it? Okay now do the opposite.

I like river bet.

His shove and show is so strange. I guess I fold vs someone I think can show up with K2, but his turn call with that hand would be pretty bad...

Would be pretty sweet play by villian if he has AK here IMO.

Shouldn't it be the opposite? If hero looks like he's about to fold and villain is bluffing, why would villain do anything to stop hero from folding? If villain was value betting and hero looked like he was going to fold, then wouldn't it make more sense for villain do some lol live stuff to convince hero to not fold? Same thing for when hero looks like he's going to call.

You folded?

Yes, definitely. Poor wording on my part.

RESULTS:

I tank folded.

I doubt it looked that way.

Whenever I see someone cutting out the call, I assume they're legitimately conflicted, and it's merely part of their decision making process.

Whenever I see someone moving chips toward the line (but not over) I assume they're trying to get a reaction. Also part of decision making process.

Did him flashing you the card change your decision??

Was gonna come in and post check river but saw I already did lol
But yea pretty easy fold vs x/shove nh if he finds a bluff here

I once observed a 2/5 hand that made it to the river. The player in position went all-in on the river for like an additional $350 into a $150 pot.

Board was A 10 9 2 5, with 3 hearts.

The OOP player is tanking forever on whether or not to call, then the guy who shoved offers to show a card and flips the jack of diamonds

Guy thinks for another 10 seconds then calls with AQ LOL!

Will never understand why he showed the J of diamonds (he had AJ)

Generally though, when someone shows a card, you can assume the other card is the best possible one to go with it.