Unlock Characters
Anguirus - Defeat "Hedorah Terrorizes San Francisco" stage.
Destoroyah - Defeat "Monster Island, The Final Battle" stage.
Fire Rodan - Defeat "Biollante Attacks Paris" stage.
King Ghidorah - Defeat "Mecha King Ghidorah Ravages Bangkok" stage.
Krystalak - Unlock all other characters but Biolantte attacks Paris, then beat it and you will get Krystaslak along with Fire Rodan.
Gigan - Beat "Ebirah Swamps" level.
Unlock Concept Art and 3D Models
Any playable character - Defeat "Monster Island, The Final Battle" level with that character
Atorgon - Defeat "Atlantis Rises From The Deep" level
Baragon - Defeat "Hedorah Terrorizes San Francisco" level
Biollante - Defeat "Biollante Attacks Paris" level
Destoroyah (NPC) - Defeat "Monster Island, The Final Battle" level
Ebirah - Defeat "Ebirah Swamps Tokyo" level
Hedorah - Defeat "Hedorah Terrorizes San Francisco" level
Jet Jaguar - Defeat "Orga Thrashes Sydney" level
Manda - Defeat "Atlantis Rises From The Deep" level
Mecha King Ghidorah - Defeat "Mecha King Ghidorah Ravages Bangkok" level
Mechagodzilla - Defeat "Cairo Is... Lost In Space" level
Megaguirus - Defeat "Biollante Attacks Paris" level
Moguera - Defeat "Mecha King Ghidorah Ravages Bangkok" level
Orga - Defeat "Orga Thrashes Sydney" level
SpaceGodzilla - Defeat "Monster Island, The Final Battle" level
The Sphinx - Defeat "Cairo Is... Lost In Space" level
Titanosaurus - Defeat "Ebirah Swamps Tokyo" level