the_randomizer Brawl Mod
the_randomizer has created a mod for Super Smash Bros Brawl that replaces Princess Peach with Krystal from Star Fox Adventures. The mod is well made and even replaces Princess Peach's golf club with Krystal's Staff. The mod has recently been improved to have much better animation and rigging, as well as replace the character portraits in-battle and on the character selection screen.
The Version 2 download has two versions of the mod (one with character portraits, one without), and detailed instructions for how to get the mod up and running.
Both downloads use 7Zip compression. To decompress them, use 7zip for Windows or Keka for Mac OS X.