Successfully complete the game to unlock the "Movie Viewer" option and a mini-game featuring Tachikoma.
Play through a level with only a handgun. At the end of the level you will be given the dual wieldable version of that handgun. You will be able to equip both of the handguns in one weapon slot.
Collect a weapon that is not in your inventory during the game, then have it equipped when completing the level. When the next level begins, it will be a part of your available inventory.
Successfully complete all Single Player mode levels with a character to unlock his or her custom weapon. After unlocking it, the weapon can be used by any character.
While playing through a level, look for a yellow/orange box-shaped robot carrying a red and white fan. If you kill him, you will be rewarded with a special Tachikoma part when you complete the level. If it was the correct robot, you will see a "Tachikoma Part Obtained" message appear once it has been killed.