He is a dentist and Rita's boss in the dental office. He first appears in "A Novel Idea", where he is constantly asking Rita to help him with the patients.
Dr. Feinstein is an old man with very little hair and a white mustache. He wears a light blue dentist gown over a white shirt. Inside of the gown's pocket, he has two pens, one is green, and the other is red.
It is implied in "Dance, Dance Resolution" that Dr. Feinstein is of Jewish heritage, as the DJ in the aforementioned episode said that he was attending a Bar Mitzvah at Dr. Feinstein's.
A Bar Mitzvah is a ceremony held by Jewish people when a person's son has reached the age of 13, and are ready to observe religious precepts, and are eligible to take part in public worship.