Directions:

Unlock Dr. Mario:
Beat the game in classic mode as Mario without using any continues and beat Dr.Mario to unlock him or play VS mode 100 times.

Unlock Mr. Game & Watch:
Beat the game in Adventure mode or Classic mode with all characters then beat him to unlock him.

Unlock Mewtwo:
All you need to do is leave the gamecube on for 20 hours, not play vs. mode for 20 hours.

Unlock Falco:
Clear the 100 Man Melee and beat Falco to unlock him or play VS mode 300 times.

Unlock Ganondorf:
Beat event #29 where Link and Zelda face Ganondorf, beat him to unlock him.

Unlock Jigglypuff:
Beat Adventure mode or Classic mode with any character. Beat him to unlock him.

Unlock Luigi:
Play Adventure Mode and on the first stage get a 2 in the second's meter. Then Luigi will appear in the next round instaed of Mario, then beat him after you clear Adventure or play VS mode 800 times.

Unlock Marth:
Use all of the original characters once in VS, Classic or Adventure mode. Beat Marth to unlock him.

Unlock Pichu:
Beat event # 37 and beat Pichu to unlock him, or play VS mode 200 times.

Unlock Roy:
Beat Adventure mode or Classic mode with Marth without continuing. Beat Roy to unlock him.

Unlock Young Link:
Beat Classic mode with Link, Zelda and 8 other characters then beat Young Link to unlock him or play VS mode 500 times.

Unlock Planet Zebes: Brinstar Depths:
Play 50 VS matches.

Eagleland: Fourside:
Play 100 VS matches.

F-Zero Grand Prix: Big Blue:
Play 150 VS matches.

Kanto Skies: Poké Floats:
Play 200 VS matches.

Mushroom Kingdom II:
Get Birdo or Pidgit trophy from Super Mario Bros. 2.

Superflat World: Flat Zone:
Unlock Mr. Game & Watch and defeat Classic mode with him.

Battlefield:
Beat All-Star Mode with any character.

Final Destination:
Clear all 51 Event Matches.

Dream Land:
Beat Target Test with all 25 characters.

Yoshi's Island:
Hit at least 1,323 feet in the Homerun Contest with Yoshi.

Kongo Jungle:
Finish 15-minute Melee with any character

Waluigi:
Beat classic with Luigi on hard with 3 lives {without continuing). If done correctly, he will challenge you. If you defeat him, he'll be yours. To play as him, hold up on the control pad and pick Luigi. Waluigi"s attacks are OK. But hiss B attack does 45 percent damage when fully charged. (Takes 3 seconds to charge!)