Hand history

Villain is unknown but i guess he is not a regular because he doesnt have full stack

First, board is dry. there is no flush draw, straight draw and else so his raise is weird.
if he has strong hand, 66, he would slowplay with it. AK,KK,AA would 4b on the preflop.
I think he can play like that with A6, because he doesnt want to see another K ott.
since there is low possibility that he bluffs at this board, I think AI vs his raise is mistake
fold is fine, i guess