Perhaps the Wii Fit Trainer was readying herself for a fighting tournament the entire time.
One of the best parts of Super Smash Bros. isn't just creating fights between Donkey Kong and Bowser; it's seeing what obscure icons Nintendo will dig up from its back catalog (so they can fight Donkey Kong and Bowser). The upcoming Wii U and 3DS versions are no exception, since Mega Man isn't the only recognizable character joining the fray. In a new E3 trailer, Nintendo revealed that the Wii Fit Trainer is also making an appearance, and she's fully capable of wiping the floor with anyone who didn't follow her workout plan. (Looking at you Kirby... I guess you've had one Waddle Dee too many.)
Traditionally, each new entry of the Smash Bros. series draws upon one left-field character to round off its roster. Super Smash Bros. Melee introduced the two-dimensional Game & Watch mascot, while Super Smash Bros. Brawl gave us the Robotic Operating Buddy as a playable fighter. The Wii Fit Trainer falls neatly into this category, but unlike the abovementioned characters, she's still fairly recognizable thanks to Wii Fit's 2007 popularity.
Of course, being a fighting game, the real question is whether the Wii Fit Trainer will be balanced enough to... you know what, nevermind. I'm sure she'll hold her own.