Successfully complete the game to unlock a movie vieweroption and a mini-game featuring Tachikoma.
Clear every mission in the 1-player story with a character to unlock that character's personal custom weapon. You can get a weapon for every character in the game, and any weapon you unlock can be then used by the other characters as well.
When playing through a level, look for a yellow/orangebox-shaped robot carrying a red and white fan. If you kill him,you will be rewarded with a special Tachikoma part when you completethe level. You will know if it was the correct robot by seeingthe "Tachikoma part obtained" message appearing afterit dies.
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When an enemy drops a weapon that you don't have in your inventory, pick it up, and make sure to finish the level while still holding it. When you start a new level the weapon will be permanently added to your inventory.
Just beat the game with any character in order to unlock a SpyHunter like minigame featuring Tachikoma, and a Movie Viewer that let's you watch all of the game's cutscenes again.