Unlock Destroyah
     To unlock Destoroyah, beat Adventure mode with Godzilla 2000.

Unlock Gigan
     Beat Adventure mode with Aguirus.

Unlock Godzilla 2000
     Complete Adventure mode with Godzilla '90s.

Unlock King Ghidorah
     Beat Adventure mode with Megalon.

Unlock Mecha Godzilla
     Beat Adventure mode with Destoroyah, Rodan and Mecha-King Ghidorah.

Unlock Mecha-King Ghidora
     Beat Adventure mode with King Ghidorah.

Unlock Orga
     Beat  Adventure  mode with all the characters, then beat the mode
once  more  on medium difficulty with Godzilla 2000. Orga will then be
selectable from the monsters menu.

Unlock Rodan
     Complete Adventure mode with Gigan.