NL25: Top two facing river checkraise

[Emperor Norton]

PokerStars No-Limit Hold'em, $0.25 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

UTG ($35.10)
MP ($8.80)
Hero ($56.85)
Button ($9.10)
SB ($28.20)
BB ($58.45)

Preflop: Hero is CO with K[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/heart.gif[/img].
UTG calls $0.25, MP calls $0.25, <font color="#CC3333">Hero raises to $1.5</font>, <font color="#666666">1 fold</font>, SB calls $1.40, <font color="#666666">1 fold</font>, UTG folds, MP folds.

Flop: ($3.75) 6[img]/images/graemlins/heart.gif[/img], 3[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
SB checks, <font color="#CC3333">Hero bets $2.75</font>, SB calls $2.75.

Turn: ($9.25) 4[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">SB bets $3.2</font>, Hero calls $3.20.

River: ($15.65) A[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
SB checks, <font color="#CC3333">Hero bets $6</font>, <font color="#CC3333">SB raises to $20.75</font>

This is such a weird line that I have a hard time putting villain on a hand. What does this read like to you guys? Only read I have is that villain is moderately aggressive.