Nintendo shared a few details about Mario Tennis Open for the Nintendo 3DS during their Nintendo Direct presentation last night. Among these was character customization for your Mii character.
In Mario Tennis Open, Camelot included over 200 items that you can collect and use to customize your Mii. Every single one of these items has a bearing on your performance in some way, whether it influences your power or spin or the way your character moves. Items can be entire tennis kits, wrist bands, raquets and so on.
You can see a demonstration of this feature in the new customization trailer embedded above. Mario Tennis Open is slated for release in North America on May 20th.