Villain is 55/25/3,0 over 21 hands

No-Limit Hold'em, $0.25 BB (8 handed) - Hold'em Manager Converter Tool from FlopTurnRiver.com

Hero (Button) ($25)
SB ($95.26)
BB ($33.98)
UTG ($10)
UTG+1 ($20.92)
MP1 ($34.87)
MP2 ($25)
CO ($25)

Preflop: Hero is Button with K, J
2 folds, MP1 calls $0.25, 2 folds, Hero bets $1, SB calls $0.90, 1 fold, MP1 calls $0.75

Flop: ($3.25) 8, K, 6 (3 players)
SB checks, MP1 checks, Hero bets $2, SB raises $5.50, 1 fold, Hero calls $3.50

Turn: ($14.25) 5 (2 players)
SB bets $14.50, Hero raises $18.50 (All-In), SB calls $4

River: ($51.25) 10 (2 players, 1 all-in)

Total pot: $51.25

Did I underestimate my opponent here and overestimated my hand? Should I have folded the turn?

I think the question you should be asking is: Should you have folded to the flop checkraise?

I would definitely continuation bet when it gets checked to me, but once I get CRd I'm folding pretty fast.

If he has two pair or a set, you're drawing almost dead. If he has Ace-high flush draw, you're a coin flip. Overall I'd say that on the flop you're in bad shape and should probably let the hand go.

This is a fold pre IMO. Sure villain looks kinda fishy/overly aggressive but you only have 21 hands on him and he already has 400bb. And even if he is a aggressive spewtard those guys have hardly ever a 3bet% > 10%...at least at full ring. So why exactly are you calling with a RIO type of hand? Sure we have position but I doubt we have card edge and since you are unsure how to play postflop in this spot not sure we have skill edge. Also you have to ask yourself whats your plan postflop. Did you play fit or fold? Villain is likely to cbet the flop so do you fold the 70% of the time you miss? And what if you hit a king or jack high board? Do you call the flop and fold the turn? How are you planning in making money?

I think you underestimated him. I like to look at fish as callers, as in I drive the boat and they call me down with marginal holdings. If they get aggressive, even if this guy is an aggrotard, I like to have a little better hand than TPGK. Once he C/Rs you you need to get out and start watching his showdowns and take notes. If he's getting out of line with marginal/poor holdings than a call like this is +EV.

I think you misread the hand, but even if the aggrofishy guy raised preflop, then it's fairly standard not to fold, the only question is whether to 3bet or flat.

Preflop is totally standard, and when this aggrofish raises me on the flop, if I'm not folding then I'm more inclined to ship it rather than call. It's pretty close as he will have a few better hands, but he's gonna have a lot of crap like worse Kx, draws, 2nd pr etc that are getting crazy. I know it's a small sample, but the likelihood is that the guy is a moron.

yeah...i did msisread the hand...sorry for that :-(... read flop as preflop.