CheatOgre.com: Metroid Prime

Change From Samus To Different Characters
To change from Samus to other characters, when you start a new file, while watching the introduction, you must hold down Z and R while entering these codes to be different dudes.

Kreana-L, Up C, Down C, Right C, Up C, Down C, L, D Up, D Down, D Right, D Up, D Down, Right C, Left D, Left C, Right D, Up, Left, Right, Down.

Live Chonzo-Y, X, X, Y, A, B, B, A, L, L, Down C, Down, Down, D Down.

Starship Pilot-Up, Down, Left, Right, Right, Left, Down, Up, Up, Down

Kraid-L, L, B, X, Y, B, X, Y, A, A, D Down, D Up, D Down, D Up, D Left, Left C, L, L, B, X, Y, B, X, Y, A, A, A, A, Y, X, B, Y, X, B, L, L, Left C, D Left, D Up, D Down, D Up, D Down, A, A, Y, X, B, Y, X, B, L, L, A, B, X, Y.

If you entered the code correctly you'll see the intro start over and the character you'll control kills Samus and takes her place.