The Chainsaw is a special weapon used only by Riggs. He obtains it by accidentally falling down an elevator shaft, and then rapidly melding together parts of the shaft. He then uses the chainsaw to move up to the next level, killing multiple Huggables in the proccess.
The Chainsaw is very much like Oliver's Katana, although stronger. Riggs swings it back and forth, effectively cutting off the heads of Huggables who are too close. When three Chainsaw ammo cartridges are picked up, the Chainsaw goes into Hyper mode. In Hyper mode, the single Chainsaw blade is replaced with three, providing a slight range increase. However, it does no extra damage whatsoever. As with all Hyper weapons, the user cannot replenish their ammo untill they are completely out.
The Chainsaw is extremely powerful, and can kill all types of Huggable in one hit.