You can change your character's outfits by tapping the four C Buttons at the character select screen. Some costume changes are limited to colors, but characters like Pikachu are also available with a hat.
Get Mew in your Pokeballs
After you get all 4 secret characters you can find Mew in your Pokeballs! It takes a little bit of time, but you'll see one. If you have the Secret Item Menu, set it to all Pokeballs and it will be easier, or go to training mode. Mew has no apparent effect on the battle.
Get Mushroom Stage
Finish the game with all original characters (all 8). The Mushroom Stage will be available under versus mode
Item Menu In Vs. Mode (US Version)
Simply fight 50 vs. mode battles. A message will come up to confirm the code has been activated.
Item Menu in the Japanese Version
Go to Vs. Mode, pick any 3 computer players and your favorite character to play. Play the "stock" mode, and set the lives to 10. Change the computer handicaps to 5. Play, if you are the victor, to you go the spoils! Enter options under vs., then go to items switch. You can choose what items will show up and how often they do. Bob-ombs galore! Infinite hammers!
Kirby's Power Loss
Everybody knows that Kirby can swallow his opponents and take on some of their powers in this game, but what if you accidentally "ate" a character you didn't want -- and you want to pursue someone else? Simple: Just press the L Button to pull of Kirby's "Hello!" taunt and he will discard whatever power he took on.
After eating another character, you can press L and Kirby will do that character's taunt.
If your playing a team stock game and you run out of lives, you can steal one life from your partner by pressing start.
Complete Bonus 1 & 2 with all characters including secret ones and a "music test" option will be on the data screen
Pikachu's Home World Advantage
Pikachu has a big advantage when playing in Saffron City.
Saffron City has metal on the ground... And metal conducts electricty!
When playing Pikachu on this level, leap into the air, and let loose with a thunderbolt while you're high in the air. The bolt will slam into the ground below. Using this technique, you can easily attack those below you from a safe distance!
Play as Captain Falcon
To unlock Captain Falcon, beat the 1P Mode in under 20 minutes. Beat him in battle and he's yours.
Play as Jigglypuff
To play as Jigglypuff, beat the game within 20 minutes.
Play as Luigi
To play as Luigi, pass the Bonus Practice 1 with at least 8 main characters. When Luigi challenges you, defeat him and he's yours.
Play as Ness (from Earthbound)
To play as ness, beat the one-player mode with any character with 3 lives, on normal difficulty. You must beat the game without continuing.
You can rotate and view the characters from different angles by entering the character bio section and holding Z. You can use the analog stick to move them around.