The Knife is a weapon in Resident Evil: The Darkside Chronicles, it is unlocked by default to all characters like the Handgun. It's used by holding the Action button and moving the motion controller, or by holding R2 and pressing either sticks on the PS3 controller.
The knife can't be unequipped nor upgraded, it's best used against small enemies that jump onto the player such as "G" larva, roaches or small spiders. It can damage Zombies if they are close enough to the player, though it won't stun them most of the time which will leave the character vulnerable.
If the player is grabbed and they successfully perform the Quick Time Event on-screen, both Claire and Krauser will perform a melee attack with the knife on their hands, both of which have different designs, with Krauser's knife resembling the same he used in Resident Evil 4, aside from this, the only other time the knife model can be seen in-game is when the partner AI uses it against an enemy.