In order to unlock Survival Mode, you must unlock the following characters in vs. mode. All characters MUST be defeated to unlock them. All locations given are the first location you encounter each enemy.
Effect - Code
Bazooka Marine - Defeat one in the Seaside Zone
Blue Demon General - Defeat one in the Cave Zone
Cluster Demon Soldier - Defeat one in the Cave Zone
Crimson Demon General - Defeat one in the Cave Zone
Knuckle Pirate - Defeat one in the Ruins Zone
Marine Corps Chief - Defeat one in the Jungle Zone
Normal Marine - Defeat one in the Seaside Zone
Pistol Pirate - Defeat one in the Jungle Zone
Sabre Pirate - Defeat one in the Jungle Zone
Sword Demon Soldier - Defeat one in the Cave Zone
World Government Agent - Defeat one in the Ruins Zone