Who is Dr. Gaghan from Shaking the Nickel Bush and what is their importance?

Dr. Gaghan is the Boston physician who initially diagnoses Ralph's diabetes and gives him his diet. Gaghan also instructs Ralph to move west and get plenty of sunshine. Ralph sends postcards to Dr. Gaghan once a week to report on his health, and he in return, tells Ralph's mother where he is while he travels through the west.