Getting Jigglypuff
Simply beat the 1P game on any difficulty level, with any amount of lives, and unlimited continues to face Jigglypuff. Then defeat him/her to use it as a playable character. Jigglypuff will challenge you in Saffron City (Pikachu's Level).

Getting Captain Falcon
Beat the 1P game in under 20 minutes on any difficulty level, any amount of lives, and unlimited continues to face Captain Falcon. Then defeat him to use him as a playable character. Captain Falcon will challenge you in Planet Zebes (Samus's level).

Getting Ness
Beat the 1P game on Normal difficulty, with 3 lives, and no continues to face Ness. Then defeat him to use him as a playable character. Ness will challenge you in Dreamland (Kirby's level).

Getting Luigi
Complete all the Break the Targets bonuses to face Luigi. When you face Luigi, you are forced to use the character that you completed the last bonus with, so don't complete your best character's bonus  until last, he can be quite tough if you are new to the game. Luigi will challenge you in the Flying Level above Peach's castle(Mario's level).

Interesting Note about the Secret Characters
Ever wondered what would happen if you hadn't unlocked any of the characters and you completed the 1P mode in 10 minutes, on Normal difficulty and with three lives? Woudl ti be Captain Falcon, Jigglypuff or Ness? The answer is Ness. The characters are unlocked in order of difficulty. If you meet the requirements to unlock all the secret characters (bar Luigi) each time, they will be unlocked in this order: Ness, Captain Falcon, Jigglypuff. If you get bored and erase the secret characters so you can get them again, their VS records will be restored once you get them back. Also you will only have to beat one Bonus 1 level to challenge Luigi.

How Hard are the Secret Characters
In all honesty, the secret characters are very easy to beat. I estimate them to be level 5-6 computers, with a handicap of 3-4, making the job of dispossing them very easy for an experienced player. If ou are new to the game, you should find it hard to beat them, but because they are on a handicap, you should be able to KO them with a SMASH ATTACK or a THROW when they are on about 70%(60% for Jiggly).

Getting The Mushroom Kingdom
Beat the 1P mode with all the regular characters (any difficulty, any amount of lives, unlimited continues) and play at least one VS game on each level. Then ou will gain access to a secret board, The Mushroom Kingdom which is a stage from the original Super Mario Brothers for the NES. I personally hate this level because the only gap on this stage is a very small gap in th middle, making edge guarging tactics pretty useless here and you can also walk off the ends of the screen.

Item Switch
Play 50 VS games to unlock the Item Switch. The Item switch allows you totoggle the different items on or off and choose the frequency at which atems appear (Nothing, V Low, Low, Normal, High, V High)

Pause the Credits
When the credits run you have control of some "cross-hairs" which you use to select the different jobs of the crew and who did them. Hitting A, will display the name of the person/people that did the job you've selected, however, the credits will continue to run. But if you hit B, the credits will stop and you can take as long as you loike to read the different people's names. Also, if you push start, the credits will run VERY fast finishing in about 20 seconds.