Though you'll get to create your own character for the Tour Mode, you'll be able to unlock old SSX characters for the Quick Play Mode. In order to unlock them, you'll have to beat their scores when you face them during the Tour Mode.

Unlockable - How to Unlock

Allegra - Beat her trick score in Tour Mode.
Skye - Beat her trick score in Tour Mode.
Tyson - Beat his trick score in Tour Mode.
Zoe - Beat her trick score in Tour Mode.