The crucial character in A Journal of the Plague Year (1722) is the first-person narrator, H. F., an established merchant in the city, whose detailed observations throughout the novel present us with an apocalyptic vision of a city and people about to disappear in the Great Plague of 1664-65.  Characteristically modest, H. F. records everything he sees with the accuracy of a video camera, and although he interacts with a few characters, his primary goal is to depict a society on the verge of extinction.  Given the preciseness with which he records all that he sees and hears, H. F. locates his own residence with enough detail to create GPS coordinates:

I liv'd without Aldgate about mid-way between Aldgate-Church and White-Chapel-Bars, on the left Hand or North-side of the Street. . . . (7)

Because the reader's belief in this account of the Black Plague relies on verisimilitude (that is, reality in the narrative), H. F.'s ability to locate events exactly and re-tell precisely the conversations and speeches that he hears throughout his walks establishes a framework for the reader's experience--in other words, the readers recognize the place and the people because they have themselves been there.

Arguably the second most important character is H. F.'s older brother, who advises H. F. to abandon London:

I had an Elder Brother as the same time in London. . . . he was for my retiring into the Country, as he resolv'd to do himself with his Family; telling me . . . that the best Preparation for the Plague was to run away from it. (9)

After convincing himself that his brother is right, H. F. resolves to flee London, but then decides to stay essentially because he reads Psalm 91:7-10, "I will say of the Lord, He is my refuge, and my fortress, in him will I trust. . . ."  Fortunately for us, H. F., despite the reasonable arguments his brother has used to convince him to leave London, ultimately relies on his profound faith, the unintended consequence of which is that H. F. is able to record a cultural disaster that wiped out substantial part of London's population.

H. F.'s friend and doctor, Dr. Heath, is another crucial character in the narrative because his advice and conversation helps keep H. F. healthy and sane enough, amidst all this horror, to continue telling the story of London's devastation:

. . . and as he was a good Christian, as well as a good Physician, his agreeable Conversation was a very great Support to me in the worst of this terrible Time. . . . (85)

Throughout his time in London, H. F. is presented with horrific and bizarre sights on a regular basis, and the touchstones by which he remains calm enough to recount this tale accurately are almost completely gone.  His friend and physician is one of those critical touchstones that allows H. F. some normalcy, some sense of the pre-plague reality.  Without Dr. Heath's practical advice about how to protect himself from the plague, H. F. would probably have succumbed to the plague long ago.

From a religious perspective, the Sexton whom H. F. encounters at the burying ground is a pivotal character because he reminds H. F. of the profoundly religious implications of the scene before them--"'twill be a sermon to you . . the best that you ever heard in your life. . . "  At the moment H. F. enters the burying ground and sees the disposal of the plagues victims, he understands the finality, the religious finality, of the plague, and, as the Sexton say, the sight becomes "a sermon."