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.
![N64 Ness :ness64: :ness64:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![N64 Kirby :kirby64: :kirby64:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![N64 Falcon :falcon64: :falcon64:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
In Melee, Mr. Game & Watch was the only newcomer that really got me, but I added Link to keep my mains even.
![Mr. Game & Watch Melee :gawmelee: :gawmelee:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Ness Melee :nessmelee: :nessmelee:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Kirby Melee :kirbymelee: :kirbymelee:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Captain Falcon Melee :falconmelee: :falconmelee:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Link Melee :linkmelee: :linkmelee:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Brawl gave me a few, Olimar, Dedede, Meta Knight, Snake, all great characters that I loved.
![Mr. Game & Watch :gw: :gw:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Olimar :olimar: :olimar:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Ness :ness2: :ness2:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Dedede :dedede: :dedede:](/styles/default/xenforo/smilies/dedede.png)
![Kirby :kirby2: :kirby2:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Meta Knight :metaknight: :metaknight:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Captain Falcon :falcon: :falcon:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Link :link2: :link2:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Snake :snake: :snake:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
While Smash 4 has taken away one of my mains, I also got many more with Pac-Man, Duck Hunt, Shulk, Robin, and Palutena.
![Pac-Man :4pacman: :4pacman:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Mr. Game & Watch :4gaw: :4gaw:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Duck Hunt :4duckhunt: :4duckhunt:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Olimar :4olimar: :4olimar:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Ness :4ness: :4ness:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![King Dedede :4dedede: :4dedede:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Kirby :4kirby: :4kirby:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Meta Knight :4metaknight: :4metaknight:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Captain Falcon :4falcon: :4falcon:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Link :4link: :4link:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Shulk :4shulk: :4shulk:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Robin :4robinf: :4robinf:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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:
![R.O.B :4rob: :4rob:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Megaman :4megaman: :4megaman:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Zero Suit Zamus :4zss: :4zss:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Samus :4samus: :4samus:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Little Mac :4littlemac: :4littlemac:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Donkey Kong :4dk: :4dk:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Diddy Kong :4diddy: :4diddy:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Mii Swordfighter :4miisword: :4miisword:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Luigi :4luigi: :4luigi:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Wendy :4wendy: :4wendy:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Ganondorf :4ganondorf: :4ganondorf:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Zelda :4zelda: :4zelda:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Sheik :4sheik: :4sheik:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Pit :4pit: :4pit:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Dark Pit :4darkpit: :4darkpit:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I'm not as strict with these but I've mostly kept them the same, they do change around some though.