Since Mario Tennis Open was launched Nintendo has been releasing QR codes for a variety of Yoshi characters, each with their own specialities and styles of play. Now that all of the individual codes have been released through various means — and potentially by some cheeky websites — there's a video that compiles all of the them in one place.
If you're looking for an edge in the online courts, or simply must have all seven Yoshi extras in your game, simply fire up the game to the file select screen and press up on the D-Pad along with Start. Your game should go into scanning mode, so follow the instructions to scan the QR codes and the characters will appear in-game. If you want to view the video and get hold of all of those Yoshi players, simply visit the official Nintendo page.
These codes may only work in Europe, and with this particular Nintendo of Europe online article we'll give you one guess who wrote it.