Who lost to Glass Joe?

Solution 1:

He has lost 99 times and has won once, in a freak accidental win against Nick Bruiser, states the Official Nintendo Magazine Punch-Out!! programmer.

Source

Solution 2:

From the Wiki:

He has lost 99 times and has won once, in a freak accidental win against Nick Bruiser, states the Official Nintendo Magazine Punch-Out!! programmer.

Looks like it's Nick Bruiser (by accidental victory).

Solution 3:

I've also heard that his win was against Gabby Jay, who himself got his one win against Glass Joe. Relatedly, I've also heard that one supposedly trained the other in the art of boxing, though it's often flip-flopped which.

I'm not sure. Official stance on Punch-Out canon has never been consistent, especially regarding Super Punch-Out!! (SNES).

As if having two titles shared between five games wasn't already confusing enough.