Mr. Bergstrom was Lisa's substitute teacher. Mr. Bergstrom took over for Ms. Hoover after she believed she had Lyme Disease and Lisa immediately took to him because he was the kind of male role model she needed in her life. However, after Ms. Hoover returned, he had to leave Springfield. A distraught Lisa ran to stop him, and Mr. Bergstrom gave her a note saying "You are Lisa Simpson". The note appears one more time in the episode "Smart and Smarter", in which Marge gives Lisa a paper that says "You are Lisa Simpson" and then Lisa says "I already have one of those".
About Mr.Bergstrom
He, like his voice, Dustin Hoffman, is Jewish