Your character can have 2 decoration equipped in his inventory. The decorations adjust your stats to prevent status effects or increase your own stats. You can switch out your decorations at any time.
Some of the items can be purchased from Jean's store, but the better items can only be manufactured with your workshop.
One of the best combinations is the Luck Blade in combination with a Critical Ring or Critical Seven decoration. The Luck Blade already has 40% increase to Critical Attacks, but when you add the Critical Ring (+20%) or Critical Seven (+7%) and you can inflict massive damage against your enemies.
There is no restriction to the types of decorations you can equip. You could equip two of the same decoration if you wanted to.