Roger Chillingworth is Reverend Dimmesdale's torturer and secret enemy. He gradually torments the sensitive reverend after he learns about Dimmesdale's affair with Hester. Roger Chillingworth initially recognizes that something is suspicious about Dimmesdale and pays particularly close attention to the reverend's affliction and mysterious illness. Given the fact that Roger Chillingworth is known throughout the Puritan village as a practiced physician, he volunteers to help the sick reverend. After Chillingworth succeeds in living with Dimmesdale, where he can closely investigate the suspicious man, he continually asks the reverend leading questions in hopes of discovering Dimmesdale's dark secret. One evening, Arthur Dimmesdale falls asleep, and Chillingworth discovers the letter "A" carved into the reverend's chest, which reveals his secret affair. Knowing that Arthur Dimmesdale is suffering from an infection and lack of medical attention, Chillingworth proceeds to enact revenge on the reverend by psychologically tormenting Dimmesdale and neglecting to care for his wounds. Hawthorne appropriately titles chapter nine "The Leech," which accurately describes the way that Chillingworth slowly drains the life from Arthur Dimmesdale. Given the fact that Dimmesdale is unaware that Chillingworth is his enemy, he is defenseless and at the mercy of the malevolent physician.

In this eNotes Educator's opinion, the relationship between Chillingworth and Dimmesdale can be summed up in one of the chapter titles:  "The Leech and his Patient."  Chillingworth, of course, is the "leech."  A leech, at that time, would have been a reference to a doctor.  It has come to be known as a negative term, but wouldn't necessarily have been known that way at that time.  Dimmesdale, of course, is the "patient."  Chillingworth, in an effort to find out the truth (and inflict the most evil possible), becomes Dimmesdale's doctor.

We must, however, delve deeper into the relationship discussed in this chapter.  A doctor is supposed to help the patient.  However, this doctor certainly does not.  In fact, this doctor has it out for his patient, Dimmesdale.  Why?  Because Dimmesdale has impregnated Chillingworth's young wife.  Chillingworth, therefore, is out for revenge.  What a perfect way to hide revenge than to be the "doctor" to a sick "patient."  Now, we also must admit that the reason why Dimmesdale is so frail all of the time is one word:  guilt.  However, there is only one physician that can heal Dimmesdale, and that is a heavenly one. 

In conclusion, it is important to remember what has already been discussed by other educators in regards to these two men's relationships to Hester (the main character):  Chillingworth is Hester's husband and Dimmesdale is Hester's love. 

Chillingworth pretends to be a concerned doctor, caring for Dimmesdale. He becomes Dimmesdale's friend and confidant, using that position to find out that Dimmesdale is Pearl's father and is suffering from his secret sin. The narrrator hints at the possibility that Chillingworth is somehow making Dimmesdale sicker. Also, it is clear that Dimmesdale is unsettled about Chillingworth's "friendship" because he admits to Hester that he hates Chillingworth.