Rufus is considered a member of the team. Actually, he often proved to be a good deal more competent than Ron, or even Kim on occasion. Ron got Rufus because he could only have a hairless pet due to his father's allergies. At first, Kim was disgusted by Rufus, but later in the show, she liked him and often even relied on his help. Rufus is largely anthropomorphic that he can stand upright, and is able to see clearly. Rufus is capable of understanding English and of abbreviated speech.
For the sake of Rufus as a character, the show ignores the reality of literal naked mole rats, who are nearly completely blind, rely upon large colony environments and specific temperatures, and cannot survive individually as pets. This has led to the disappointment of many fans who have sought a pet naked mole rat and have found that not only are they unable to be pets, but look and act very little like Rufus in real life.