Outfit can be upgraded via synthesis, attribute will raise after each level up. You can become a stronger after reaching certain level regardless of your outfit's initial attributes.
To upgrade outfits is actually to synthesize them. After a success synthesis, "+X" will appear in the outfit description, indicating its level. Place the target outfit in the main item column, then place two identical level +0 outfits into the catalyst column to start synthesizing. The higher the outfit level is, the lower the success rate will be.