If you enjoy Mario Tennis Open, you may have already seen some of the QR Codes that were released by various sources, each unlocking a uniquely-colored Yoshi.
If you’d like to add another playable character to the roster in the latest portable Mario Tennis title, access the hidden QR-scanning feature from the Profile Select screen by simultaneously holding Start and pressing “up” on the d-pad. From the next screen, scan the strange black and white “code” in the image below:
Voila! You should now be able to play as Metal Mario in Mario Tennis Open. Enjoy!