This option in Smash Run allows players to modify their favorite character. They can change their characters' equipment, specials, and equipped Powers. Each character can have up to 10 customized versions.
Equipment
Each character has three equipment slots. The equipment are color-coded; they have the same color of the stat they increase. Take note that each equipment type increase one stat while decreasing the another. Orange equipment increases attack but usually lowers defense. Blue equipment increases defense but lowers speed. Green equipment increases speed but lowers attack.
You're free to try any equipment you want but you should consider what could benefit your selected character's abilities. There are characters that are already quick, sturdy, or powerful by default; losing a few points in the attribute they already excel at can be a good tradeoff for equipping an item that can provide a significant boost and bonus effect.
Some equipment have Bonus Effects. To view the description of their bonus effect they provide, you can select them then press X to view the tooltip in the lower part of the upper screen.
Unwanted equipment can also be sold for Gold. Be careful not to sell your valuable equipment!
There are certain uncommon and unique equipment that can only be equipped by certain characters. They'll have special icons and will only be listed in the equipment screen of the character they're compatible with. To see the full list of these equipment, please click this link.
Specials
Each character has four default Special moves that can be activated depending on the stance. (Neutral, Side Special, Up Special, and Down Special) As you play various modes in the game, you'll have the chance to collect alternate specials for your characters. You can change them in this menu.
The upper screen will display the special's description while the lower screen shows the available alternate specials that you can select.
As you play various modes in the game, you'll have the chance to collect more Powers that you can equip to your characters in Smash Run. Each of this powers have the corresponding Weight. Every character has a maximum weight limit. The general rule is that heavier characters will be slower but have a higher weight limit, enabling them to carry more "heavier" powers. Light characters in the other hand are faster but they can't carry that much powers because of their small weight limit.
There are also powers that have identical names but with different number of uses or charge. Powers with more uses will weigh more. For the list of all the powers available, please check this link.
Once you've successfully modified your favorite character, you can now review the changes made and go to Training to test your equipment and specials setup.
You can also use Mii Fighters in Smash Run. Like the characters in the game rosters, you can modify their equipment, skills, and Powers. In addition, you can modify their Headgear and Outfit. Mii gear are rare drops in Smash Run and other non-Smash modes. Keep playing and you'll eventually unlock more customization options for your Mii Fighter.
You can create your own Mii Fighter by using saved Miis in your 3DS. If you don't have other Miis, you can use the default "guest" Miis on the left side of the Mii selection screen. Should you want to create your own Mii to use it as a template for your Mii fighter, you need to do it outside the game, in the 3DS's main menu.