The glitch characters randomly appear through standard bugging. If you use a Gamecube keyboard, you can alter the game code to an extent. It doesn't always work, though, and there is no guarantee that your game will remain functional. Even I don't know how to work it properly. If you can work this system, then you can get the glitch characters more easily. By the way, go to your names list to get the glitch characters. Here's sopmething you can try. If you can get a character to eat a random enemy, which appear from crates from time to time, then you might get some visible code on the title screen. The most common one starts with 1856, then has 4 random letters, then 6 random numbers, then a 0. Enter the letters in the name list, and you should get a new character, item or arena. I cannot guarantee anything, though.
10000 posts at last! w00t! Okay, I'm off into a quiet retirement. Don't bother me.