SB is Loose-Agressive

Party Poker No-Limit Hold'em, $ BB (6 max, 5 handed) converter

BB ($37.57)
UTG ($103.17)
Hero ($41.65)
Button ($53)
SB ($123.46)

Preflop: Hero is MP with K, T. SB posts a blind of $0.5.
UTG calls $1, Hero raises to $3, 1 fold, SB (poster) raises to $5.5, 1 fold, UTG folds, Hero calls $3.

Flop: ($14) 5, K, 6 (2 players)
SB checks, Hero bets $5, SB calls $5.

Turn: ($24) 4 (2 players)
SB bets $10, Hero calls $10.

River: ($44) 6 (2 players)
SB bets $21, Hero ????