If StreetPass Data for Kirby: Triple Deluxe is found on your 3DS, you'll unlock 2 new abilities and 1 new stage.

Unlockable	How to unlock
Beetle Ability	Have StreetPass data for Kirby: Triple Deluxe on your 3DS.
Bell Ability	Have StreetPass data for Kirby: Triple Deluxe on your 3DS.
Waddle Dee Train Tracks	Have StreetPass data for Kirby: Triple Deluxe on your 3DS.