Successfully complete the indicated task to unlock the corresponding costume.

Achilles' Original outfit (at manor): Successfully complete all Achilles' Homestead missions and interactive conversations.
Altair's robes (at manor): Get a 100% Sync on all story missions.
Assassin's robes: Start Sequence 6. Different colors can be purchased from general stores.
Baltimore outfit (at shops): Start Sequence 9.
Boston outfit (at shops): Start Sequence 6.
Captain Kidd's outfit (at manor): Collect all 24 Peg Leg Trinkets, 4 Map Shards, and solve the riddle on Oak Island.
Charleston outfit (at shops): Start Sequence 6.
Jamestown outfit (at shops): Start Sequence 6.
Kanien'kehaka outfit (at manor): Collect all feathers.
New York outfit (at shops): Start Sequence 9.
Philadelphia outfit (at shops): Start Sequence 9.
Prisoner outfit: Start Sequence 9.