Tweak is a cat in the novel Final Destination: Looks Could Kill. He is owned by Ruby's boyfriend, although Ruby is seen taking care of him. Tweak didn't make a noticeable appearance until Sherry arrived to Ruby's apartment after the Coral Clipper accident, which mangled her face horribly. While there, a mouse (which was inside a box Sherry brought over) escaped and ran inside Ruby's dark room, with Tweak running after it. Ruby chased after the cat and got angry at him once it got on a shelf and knocked over several jars of corrosive fluids. Tweak and the mouse escaped the room while Ruby was locked inside. She later died after going into spasms and landing in the pool of chemicals, which eat her face away.
Since Tweak is partially responsible for the death of Ruby, he is most likely a Servant of Death.