As you meet people with the game on Street Pass, you'll be able to find new Legendary Pokemon on different areas.

Unlockable	How to unlock
Azelf [Shimmering Lake: Lake Area]	Meet 10 People.
Celebi [Everspring Valley: Forest Area]	Meet 60 People.
Jirachi [Firebreathing Mountain: Tower Area]	Meet 40 People.
Manaphy [Rugged Flats: Beach Area]	Meet 20 People.
Mesprit [Soothing Shore: Lake Area]	Meet 5 People.
Mew [Sunny Seashore: Factory Area]	Meet 80 People.
Phione [Rugged Flats: Beach Area]	Meet 20 People.
Shaymin (Land Forme) [World Axle Underground 2F: Ice Area]	Meet 100 People.
Shaymin (Sky Forme) [World Axle Underground 1F: Forest Area]	Meet 120 People.
Uxie [Sun-Dappled Bank: Lake Area]	Meet 2 People.
Victini [World Axle Underground 2F: Tower Area]	Meet 150 People.