think about this:
there is a ball fortune in chapter 3 that says, "don't let your guard down, that chair is going to kick your ass!"
the chapter 3 ball fortunes are all about chapter 3, which means the chair ass-kicking necessarily happened in chapter 3.
the only ambiguous and challenging fights in chapter 3 are the knight and ERAM
the knight is the only genuine mandatory ass-kicking, and ERAM is referential enough to the knight plot (with the shadow mantle's implied owner and the "we both have work to do" line) that it's very likely that the knight was the one on the other end.

so who does that leave for the chair-based ass kicking? the knight. the knight is friend.