Common marginal situation

[dmtpt]

Ultimate Bet 0.25/0.50 Hold'em (9 handed) internettexasholdem.com

Preflop: Hero is SB with A[img]/images/graemlins/diamond.gif[/img], T[img]/images/graemlins/spade.gif[/img].
<font color="#666666">5 folds</font>, <font color="#CC3333">CO raises</font>, <font color="#666666">1 fold</font>, Hero calls, <font color="#666666">1 fold</font>.

Villian is entirely unknown. Bad decision to defend? If so, is it OK w/ATs? AJo?

Flop: (5 SB) 3[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/spade.gif[/img], 6[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">CO bets</font>, <font color="#CC3333">Hero raises</font>, <font color="#CC3333">CO 3-bets</font>, Hero calls.

Turn: (5.50 BB) 8[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">CO bets</font>, Hero ...

Does an unknown have me beat here enough to fold? Would a new guy at the table overplay something like KK, or an A-rag suited steal attempt?