July 18 Update
Metal Mario for North America released! That wraps up all the QR characters! Thank you for being a part of this guide and letting us know your thoughts in the comments! However, the comments section will close tomorrow, so please get in your last comments today!
Mario Tennis Open is full of unlockables, and downloadable QR characters and costumes are among them. Keep checking back to this page as we continue to update with the latest QR code unlockables!
To unlock the characters and costumes in the game, one must go to the file select menu of the game, and hold down both the START button and Up on the D-Pad. This will activate a QR camera mode in which you can use the Nintendo 3DS camera to scan the QR codes and instantly unlock the character or costume. Check out the currently available codes below and start scanning! If you live in an area where you have a different flag, use the flag that the country is suited as a whole. For example, if you live in Canada, use the United States of America flag.
Light Blue Yoshi
Thanks to TeaStrife for the Black Yoshi scan! Thanks to Joe for the Pink Yoshi QR Code.