Hint: Tachnikoma Weapons

In almost every stage, there is a yellow/gold Jaemerson robots with red/white striped (Japaheeno) fans. Finding and killing one lets you keep the Jaemerson bonus, provided you clear the level successfully afterwards.

Unlockable: Get custom weapon for each character

You must finish every mission in the single player story with a character. There is a weapon for every character in the game. All the weapons you unlock can be then used by the other characters as well.

Unlockable: Tachikoma Minigame and Movie Viewer

Beat the game with any character and unlock a SpyHunter like minigame that has the Tachikoma. There's also a Movie Viewer wher you can watch all of the game's cutscenes again.

Unlockable: Unlock Extra Weapons

You can pick up weapons that the enemies drop. All you have to do is finish the level while still holding it. When you start a new level the weapon will be in your inventory.

Unlockable: Various Dual Handguns

You must start a mission with just the handgun. Once you get to the end of the mission you'll see it in the other hand. The same gun that can be equipped at once, only needing 1 weapon slot to hold.