Wii Fit Trainer is the new Captain Falcon. She's the character that NOBODY saw coming, and if you knew her from the game she came from (Wii Fit) you NEVER would have imagined her in a fighting game of any sort, much less Super Smash Bros. But then she arrived in Smash and quickly became a fan favorite.
Way back when, Captain Falcon's appearance was restricted to his car, the Blue Falcon, which he never exited. The two F-ZERO games were merely racing games, games where it could be argued that you played as cars, not the characters that drive them and never leave the cockpit. So when Captain Falcon showed up in Super Smash Bros, it was a WOW moment. Had the Internet existed at the time in the way it does today, we'd be getting comments like "But he's a racing game character, he never leaves his cockpit or anything else in the F-ZERO games, why is he here?", but sure enough, Falcon Punch became the meme it is and Falcon forever a fan favorite.
Melee of course had Mr. Game and Watch, that character that really doesn't sound like he would make a ton of sense, but sure enough, he's also a bit of a favorite.
Brawl had R.O.B., a character who really isn't even a character so much as he is a controller (ok, he was in Mario Kart DS that one time). Nobody would ever truly expect a guy like that, but sure enough, here we are, anxiously awaiting his return!
Captain Falcon, Mr. Game and Watch, R.O.B., and Wii Fit Trainer. No one asked for them, no one expected them, people might have even scoffed at the very idea... but now we just can't imagine Smash without them.