If you have trophy/save data from Assassins Creed, Assassins Creed 2, and/or Assassins Creed 3 on your system, you will receive the main protagonists costume from those games.

Robe	How to unlock
Altair Robes	Have Assassins Creed data on your system.
Cat Outfit	Go to any general store and buy the animal pelts needed to craft it.
Connor Robes	Have Assassins Creed 3 data on your system.
Ezio Robes	Have Assassins Creed 2 data on your system.
Hunter Outfit	Go to any general store and buy the red howler monkey skin (5,000R) & white jaguar pelt (6,000R). Craft the outfit.
Mayan Outfit	Collecting the 16 Mayan Stones that you get from finishing each of the Mayan Stelae missions, followed by opening up the secret door in Tulum.
Stealth Outfit	Finish half the Assassin Contracts in the game.
Templar Outfit	Collect (mine)keys from finishing each of the 16 Templar Hunt missions, followed by opening up the cage in the Great Inagua hideout.