Tabuu

A mysterious entity that attempted to drag the entire world into his dimension known as Subspace. He is humanoid in appearance and has a number of powerful attacks. His most powerful of these is an area of effect attack that causes an instant KO. Tabuu is a dangerous opponent for the unprepared.

This is how you scan the code, and I'm quoting Nintendo here:
1: Visit the Able Sisters store on Main Street and talk to Sable (the hedgehog at the back of the store) about using the big sewing machine next to her. If the machine hasn't appeared in the store yet, come back and talk to Sable everyday!
2: Choose 'Read a QR Code' from the menu and then select which of your ten current custom designs you wish to delete to make room for the design you're scanning.
3: Use the outer cameras of your 3DS system to align the on-screen frame with the QR Code to register the design. Note that pro designs require all four QR Code patterns to be scanned before they can be saved, so follow the on-screen instructions.

Animal Crossing and is copyrighted to Nintendo.
Super Smash Bros is copyrighted to Nintendo/Sora Ltd.