Mr. Hat is Garrison's hand puppet that he uses to express his negative (and gay) feelings. Mr. Hat makes Mr. Garrison do terrible things like attempt to kill Kathie Lee Gifford and write gay novels about penises. During the second season he was kidnapped and briefly replaced with Mr. Twig. He returned shortly after, and helped Garrison through many a life-change...including coming out of the closet. Garrison replaced Mr. Hat with Mr. Slave when he moved up to teach the boys in 4th grade (in "The Death Camp of Tolerance"), feeling that the boys were now "too old" for his puppet-in-crime. However, Mr. Hat did make a big return in the "200"/"201" saga, helping Cartman find out the truth behind his real father.
Season 2: Conjoined Fetus Lady
Nurse Gollum believes the citizens of South Park citizens are a bunch of freaks.
Season 1: Cartman's Mom Is a Dirty Slut
Cartman learns that many people have slept with his Mom, so he may never know who his father is.