This will only work if your characters are not yet equipped with armor. This is very simple, all you need to do is buy just one armor for the character you want this to work for and make sure he has no other armor equipped. Once you have bought the armor of your choice save your game, then equip the armor and then load your game. You will notice that your characters defense stays the same as if he has still got the armor equipped but he has not, then simply repeat the process and there you have it, infinite defense increase.