Who is Dr. Theodore Sephanides from My Family and Other Animals and what is their importance?

Dr. Theodore Sephanides meets Theodore when Gerald rushes into George's home with a question about an insect habitat he has discovered. Theodore is a nature-lover like Gerald, and George considers him an expert on everything. Over the course of the story, we learn that Theodore begins to spend one day a week visiting Gerald and his family, and he becomes a favorite of them all.