lol
As noted, all you need for ALMOST all characters are their PCS files.
However, the only exceptions are Bowser, Sheik, Zelda... the transformation characters, basically.
So, in order to make said ^ characters work properly, you simply put the PCS and PAC file together. (I'm assuming you're using the SD Loader.)
Let's take Bowser for example... You have an edited green-colored Bowser, "FitKoopa00.pcs".
Simple as this:
FitKoopa00.pcs
FitKoopa00.pac
Basically, the PAC file is named EXACTLY the same as the PCS file (minus ending in .pcs, of course
).
FitWario07.pac
FitWario07.pcs
FitZelda01.pac
FitZelda01.pcs
FitPokeLizardon02.pac
FitPokeLizardon02.pcs
Simple as that.~
(You can also supply the PAC files with the PCS files in the SD Loader to make characters appear in various other modes, like the Subspace Emissary mode, or Boss Battles... Modes outside of normal Brawl-ing mode.)