Buster was a gift for Andy at the end of the first movie.  He was never seen however until the second movie.  In the comic book Buster almost foils a rescue attempt to save Buzz Lightyear.  he is distracted by Weezy the Penguin. In the third movie he is shown to be quite slow because when Woody calls him Buster just falls down. He is also shown to have a good relationship with Andy despite his old age.