Ultraman Nexus Cheats (PS2)

To unlock hidden fighters of the game like Ultraman Noah, play the survival mode of the
game with the 2 enemy characters that are available in the upperright of the character
selection menu. 

To unlock the third Nexus, play the game with the character which is in the bottom of the
middle row.

As normally you can play as the 3rd Deumantis when you are defeated. Successfully complete
the game once and after that you are going to have the option to select between the two
characters.