My "mains" are the characters I love the most. I have a large number because I only add to them with each game, and I have only ever lost one (with Snake being cut). I'm sorry if this doesn't fit someone else's specific criteria, but this is how I do things.
In Smash 64, Kirby was the character I went with from the start, once I unlocked Ness and Falcon, they fell into my mains.



In Melee, Mr. Game & Watch was the only newcomer that really got me, but I added Link to keep my mains even.





Brawl gave me a few, Olimar, Dedede, Meta Knight, Snake, all great characters that I loved.









While Smash 4 has taken away one of my mains, I also got many more with Pac-Man, Duck Hunt, Shulk, Robin, and Palutena.












So yes, I have thirteen mains. I don't think that's a lot, considering the size of the roster. Am I great with all of them? No, Captain Falcon is probably the worst of my mains, while I'm at my best with Olimar, but I still enjoy playing as them all. Had Snake not been cut, I would have fifteen mains, as to keep things even, R.O.B. is the next in line to be bumped up like Link was.
Though I have Ness as my "main main," that is mostly for consistency, as he's been in every game since the beginning. If I had to pick a single character to be my main (best with, use the most, is my favorite, etc) then it would be Snake, but of course...
Of course, I also have secondaries:















I'm not as strict with these but I've mostly kept them the same, they do change around some though.