SB vs BB fight. Do I have to call this river bet ?

[King_Arthuro]

PokerStars No-Limit Hold'em, $0.50 BB (5 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Button ($30.15)
SB ($48.50)
Hero ($64.85)
UTG ($25.50)
MP ($60.50)

Vilain is a new one. Zero read.

Preflop: Hero is BB with A, 7.
[color=#666666]3 folds</font>, [color=#CC3333]SB raises to $1.5</font>, [color=#CC3333]Hero raises to $5.5</font>, SB calls $4.

Flop: ($11) 6, A, 4 [color=#0000FF](2 players)</font>
[color=#CC3333]SB bets $7</font>, Hero calls $7.

Turn: ($25) T [color=#0000FF](2 players)</font>
SB checks, Hero checks.

River: ($25) 9 [color=#0000FF](2 players)</font>
[color=#CC3333]SB bets $20</font>, Hero folds.

Final Pot: $25

Thoughts about flop : Do I have to raise and fold to a reraise ?
River : As I 've never represented the A, is this a call ?

Thx for opinions

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