In my game when I went into the outfits I found I had the previous Assassin's outfits in with the rest. I assumed this was because I had played the previous games (didn't complete Assassin's Creed 3) and they got unlocked because the game picked up their save games.
When my dad went to play the game and went to his outfits he didn't have any and he's completed Assassin's Creed 3. the only difference between his profile and mine is that I have an x-box live account which I use to update the game on his 360.
I am wondering, what unlocks the Previous Assassin's outfits?
EDIT: To clarify (as I didn't think of it) I have never used UPlay and always skipped any signup/log in step.
Best Answer
Some of the outfits (namely the iconic outifts from AC 1-3) are unlocked if you have those games linked to your UPlay account. This might explain why you have them and your Dad doesn't.
However If you have any XBOX/PS3 saves from the previous AC games, the Altair/Ezio/Connor robes will be unlocked and ready to wear when Edward has access to his quarters on the Jackdaw after the Raise the Black Flag mission. The robes are equipped via UPLAY and cannot be worn offline.
So to answer your question more clearly, these outfits are unlocked on console by playing previous games. AC 4 will read you save games and unlock these outfits, but they cannot be worn/unlocked offline - which is why your dad does not have access to them.
EDIT: You can get these by linking these games or by linking 1 - 3 games in the series (i.e. Assassin's Creed Brotherhood and Revelations also count).