Perform the following actions to unlock additional costumes for Ezio.
You can wear costumes by equipping them in the Inventory menu.

Raiden                         - Get a Bronze or better in all Virtual
                                 Training Challenges.
Florentine Noble Attire        - Purchase for 2 Uplay tokens
Altair (from Assassin's Creed) - Purchase for 2 Uplay tokens
Altair's Armor                 - Purchase for 2 Uplay tokens
Desmond Costume                - Beat Sequence 8 with 100% Synch