Rufus is a Red Crab. He does heavy damage to aliens that you lure to the ground. He is unlocked on Level 2-3.
When you lure an enemy to the bottom of the tank, Rufus will proceed to swipe at it with his claws, doing heavy damage. He is very good against Destructor, as he attacks him immediately. He is one of the 3 robot killing pets, the others being Itchy, and Gash.
Rufus is relatively useful against robot, but there are better pets. If you want a pet to better fight aliens, pick Gash. Otherwise, there are plenty of other pets to choose from.
Rufus used to be a dance instructor, teaching people to do his patented pop and lock arm routine until his studio burned down. Then Rufus became a mercenary for hire.