Considering my personal history...
My main since Melee, Jigglypuff was one of my favorite Pokémon due to the one wandering in the anime. At the time I've considered maining
and
as well, but they didn't feel well to me. Before Jigglypuff I briefly played with
because they were two and were nice.
As a huge fan of Pikmin, Pokémon and Kirby, and hyped by ROB for his presence in Mario Kart DS, I mained them while I was waiting for unlocking Jigglypuff in Brawl. Later I used them in those times I didn't feel the Puff, and the one that remained in Sm4sh (I prefered Squirtle to Charizard D: ) are among my secondaries right now.
Discovered them in the last period of Brawl prior to the demo of Smash 3DS. I was looking for someone who was able to kill the bosses on Very Hard mode, and I thought their smashes (dair of Lucas in particular) were what I was looking for, and I succeded with Lucas. Ness feels good to me in the new Smash, so now he is my third-in-command. Also, I played Earthbound on the Wii U and I'm playing Mother 3, so I like the series.
I always kept far from Pikachu, because, while I don't hate this Pokémon, I prefer others and he has too much appeal. Also, back in Melee, where he was my first character used, I wasn't able to optimize his recovery. So that's why I haven't considered him as a main... until the demo. Here I discovered his true potential, and since he was one of only five characters, I had one entire month to train him, and now he is my second-in-command after Jigglypuff.
I also used them in the demo. Villager is in because he is from Animal Crossing, and that's enough. Megaman is in because he is funny.
They are the newcomers who impressed me the most. I'll wait for the Wii U version to truly optimize them.
I considered him as a secondary in the past. Now he is one of them because once I met a G&W main in For Glory and I experimented a mirror match. He is fun to use.
Like him, but never felt his Smash representation. Since now he seems good, I am experimenting him.