As one of the primary heroes of Dracula, young, handsome solicitor Jonathan Harker is perhaps the best representation of the “perfect” English gentleman, one who's thrust into a situation beyond his imagining and forced to literally fight to the death to save his love and eventual wife, Mina Murray.
The novel indeed starts with Harker and his trip to Castle Dracula. From minute one, Harker is tested, beginning with his leaving for the castle on the eve of St. George's Day, which as told to him by the hotel purveyor is when “all the evil things in the world will have full sway.”
Forcing a rosary with a crucifix upon him for safety, the stage is set for the true horror Harker is to face when imprisoned by the devil and confronted with a rapture of emotion, superhuman occurrences, Dracula's brides, humiliation, and his own survival, escape, and return to London.
Harker in many ways epitomizes the average man whose proper belief system and morals are challenged to the very brink of his sanity. Complying to some measure of internal revenge, it is Harker who first steps up to the plate to volunteer his services to Van Helsing in destroying Dracula. Indeed it is Harker who in the final confrontation at the Castle slits Dracula's throat before Quincey P. Morris stabs the fiend with his Bowie knife.
What's of particular interest about Harker is that his is the first character that in many vampire films is either primarily featured or quickly eliminated. For example, as in the novel, Harker is the one who visits Castle Dracula in the 1922 film Nosferatu and the 1992 Bram Stoker's Dracula, but in the 1931 Dracula, based primarily on the Hamilton Deane/John Balderston stage play, it is Renfield and not Harker who makes the journey and becomes Dracula's servant.
Still other films use Jonathan Harker as well as the other characters in various measure. In Hammer's seminal 1958 film Horror of Dracula starring Christopher Lee, Harker makes the trip to the Count's castle, but not as the simple librarian he claims to be. Instead, he's working in league with Van Helsing (Peter Cushing) and arrives with the intent to kill the fiend.
Unable to accomplish his task, he becomes a vampire himself and is very quickly staked by Van Helsing. The rest of the story revolves around Arthur Holmwood, his wife Mina, and his sister Lucy. Dr. Seward is but a minor character, and Renfield and Quincey Morris are nonexistent.