It may be used on allies who do not attack often, or to avoid the defense reduction effect of another equipped medal, however, it should be remembered that it also reduces attack.
How to Acquire
There are 2 Defense Exchange medals available. It can be found at the earliest right when Chapter 4 starts, by using the Mysterious Piece, given by Professor Neolith, to unlock the Cave of Trials and complete the 30th wave.
Rewarded by completing wave 30.
When Chapter 5 starts, this medal can be bought from Shades in the Underground Tavern for 5 crystal berries.