Where does the Doctor fit in?

For a detailed look at each of these characters, simply follow the link provided below to access Gradesaver's summary of the Prologue. In this summary, you will find the order (from highest to lowest) or status of each of the pilgrims. I assume that this is what your question refers to as far as "fitting in." Chaucer's pilgrims represent the different rungs of English society. The summary will supply you with this information in order of importance. 

A Doctor of Medicine, clad in red and blue, cannot be matched in speaking about medicine and surgery. He knows the cause of every illness, what humor engenders them, and how to cure them. He is a perfect practitioner of medicine, and he has apothecaries ready to send him drugs and mixtures. He is well-read in the standard medical authorities, from the Greeks right through to Chaucer's contemporary Gilbertus Anglicus. The Doctor, however, has not studied the Bible.