Yes, you can unlock legendaries. Here's a list of legendary Pokemon you can get:
Tornadus - Cove Area: Seasong Beach - Befriend him by beating his Tag Race. (Only available after you beat the game)
Victini - Crag Area: Rockridge Fort - Befriend him by rescuing him from Wish Park
Thundurus - Crag Area: Colosseum - Befriend him by beating his Tag Race. (Only available after you beat the game and befriended Tornadus)
Landorus - Arcane Area: Lakeshore - Befriend him by battling. (Only available after you beat the game and befriended Tornadus and Thundurus)
Reshiram - Wish Park, Access in Arcane Area: Lakeshore - Befriend him by battling. (Only available after obtaining 20 feathers)
Zekrom - Wish Park - Befriend him by battling. (Only available after you obtained the item from Reuniclus after befriending Landorus
Darkrai - Wish Park - Befriend him by defeating Reshiram/Zekrom.
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