In order to receive the "Dot Eater!" ranking upon completing a level, do not fire the Ikaruga even once. This means that you can't kill normal enemies or the boss of the stage, but rather you will have to dodge/absorb enemy fire in order to complete the level. This ranking does nothing but add extra challenge to the game.
Chain Counter
To toggle on a chain counter which appears beneath the Chain Status box, press Z at anytime during gameplay.
Extra Continues
For every hour of playing your amount of continues will go up to a maximum of 9 continues/6 hours. After 7 hours, you'll have unlimited continues.
Make the pause menu disappear
While playing the game, press start and a pause menu will come up. Press and hold X and Y together to make the menu disappear.
Unlock Gallery 1
To unlock Gallery 1, beat Trial Mode without continuing. Or, play the game for more than five hours total.
Unlock Gallery 2
To unlock Gallery 2, beat the game on any difficulty. Or, play the game for more than ten hours total.
Unlock Game Mode Option
To unlock the Game Mode option, beat the game on Normal without continuing. Or, play the game for more than twenty hours total. The option can be found in the Appendix menu.
Unlock Sound Test
To unlock Sound Test, beat the game on Easy without continuing. Or, play the game for more than fifteen hours total. Sound Test will be available in the Appendix menu.
View different demos
In order to see different demos of how a stage is played out in Ikaruga, change the monitor type settings before going into demo mode. The different settings will show these types of demos:
Vertical 1&2: These settings will show increased difficulties.
Horizontal 1: This setting will show beginning tactics.
Horiztonal 2: This setting will show more advanced tactics.
Horizontal 3: This setting will show advanced tactics or bullet eater tactics.
Bullet Eater...