Proctor is the protagonist of the play. He appears in nearly every scene. His relationships to Mary Warren, Abigail, and his wife Elizabeth are telling regarding his character.
Proctor is Mary Warren's employer and, importantly, also functions as her moral adviser in the second and third acts of the play. Mary Warren attempts to accept Proctor's advice and do the right thing (exonerate Elizabeth), but she fails in this effort, betraying both John and Elizabeth Proctor.
Proctor is generally impatient with Mary Warren, viewing her as weak-minded and inadequately dedicated to her job. Proctor's views of Abigail and his relationship with stand in contrast. Where Proctor's staunch morality is exhibited in his relations with Mary Warren, his weaknesses are expressed through his relations with Abigail.
This is true as well of Proctor's relationship with his wife. In dealing with both Abigail and Elizabeth, we see Proctor struggle to regain a sense of pride, honor and goodness. He has failed his wife and blames himself. However, as Abigail continues to insist that they rekindle their affair and Elizabeth continues to punish him, Proctor becomes an accuser of both women.
Embattled as he is in a drama of his own construction, Proctor is hesitant to engage in the larger drama of Salem (the witch trials). Yet, his relationships drag him into that larger fight.
Ultimately, Proctor has to stand alone as he walks to the gallows. At this point he has severed relations with his former mistress and has found self-forgiveness and forgiveness from his wife.