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In the novel 
A description of Renfield from the novel:
R. M. Renfield, aetat 59. Sanguine temperament, great physical strength, morbidly excitable, periods of gloom, ending in some fixed idea which I cannot make out. I presume that the sanguine temperament itself and the disturbing influence end in a mentally-accomplished finish, a possibly dangerous man, probably dangerous if unselfish. In selfish men, caution is as secure an armour for their foes as for themselves. What I think of on this point is, when self is the fixed point the centripetal force is balanced with the centrifugal. When duty, a cause, etc., is the fixed point, the latter force is paramount, and only accident or a series of accidents can balance it. — From Dr. John Seward's journal
He is an inmate at the lunatic asylum overseen by Dr. John Seward. He suffers from delusions which compel him to eat living creatures in the hope of obtaining their life-force for himself. He starts by consuming flies, then develops a scheme of feeding the flies to spiders, and the spiders to birds, in order to accumulate more and more life. When denied a cat to accommodate the birds, he eats the birds himself. He also changes his ideas to accommodate Mina Harker by quickly eating all flies and stating that it was an old habit. Doctor John Seward diagnoses him as a "zoophagous maniac", or carnivorous mad man.
During the course of the novel, he is revealed to be under the influence of Count Dracula. The vampire, whose abilities include control over animals such as rats, bats and spiders, comes to Renfield with an offer: if Renfield worships him, he will provide Renfield with an endless supply of food.
However, when confronted by Mina Harker, the object of Dracula's obsession, Renfield suffers an attack of conscience and begs her to flee from his master's grasp. Renfield is consumed by his desire to keep Mina safe, begging Seward and the others to allow him to leave lest he feel guilty for her fate. When he is denied by Seward, Renfield tells the group of vampire hunters that "[he] warned them!" When Dracula returns that night, Renfield is again seized by his conscience. He remembers hearing that madmen have unnatural strength, and so attempts to fight Dracula. Renfield's strength leaves him after looking into Dracula's eyes, and Dracula throws him to the floor, severely injuring him.
The vampire hunters enter the room shortly afterward, and through an emergency surgery Van Helsing manages to prolong Renfield's life. Renfield tells his story to the vampire hunters who rush to help Mina, and leave him lying on the floor. He lives for only a few moments more before succumbing to his injuries, dying alone.
In other media 
Film adaptations of the novel, if they include Renfield, have a tendency to expand his role, making him a long-standing servant of the vampire Count, often depicting his mania as a result of falling under Dracula's influence, rather than as a pre-existing condition that made him vulnerable to it. Tod Browning's 1931 film, for example, conflates the character with that of Jonathan Harker, making Renfield (played by Dwight Frye) the real estate agent who is sent to Transylvania and falls under Dracula's (Bela Lugosi) power. The 1922 silent film Nosferatu presents Alexander Granach as a Renfield similar to that of the novel, but gives him the name Knock, and in a deviation from the novel, survives only to be caught and trapped in prison where he is unable to help Count Orlok, his master, escape the morning sunlight. Nosferatu also differs from the novel in making Knock the real estate agent who employs Harker. This deviation also appears in Nosferatu the Vampyre, the 1979 remake of the 1922 film. Here, Renfield (portrayed by artist-writer Roland Topor) escapes from the asylum after being committed for biting a cow and helps Dracula spread a plague in his town. Whether or not he is re-captured is unknown. In 1966 Hammer film Dracula: Prince of Darkness the character of Renfield played by Thorley Walters is renamed to Ludwig (because the film is set in German-speaking environment).
In Count Dracula, Klaus Kinski (who portrayed a Count Orlok-style Dracula in Nosferatu the Vampyre) played Renfield as mute. Kinski's own Nosferatu the Vampyre (1979) has its own Renfield, actor Roland Topor, who, as in the original, is Jonathan Harker's employer who goes insane before Dracula arrives.
The BBC version of Count Dracula (1977), starring Louis Jourdan in the title role, includes Jack Shepherd as a sympathetic Renfield in a prominent role which highlights his relationship with Mina. The 1979 film Dracula, starring Frank Langella in the title role, has Tony Haygarth playing "Milo" Renfield as an unkempt workman who in enthralled by Dracula while he is unloading the boxes as Carfax. Francis Ford Coppola's 1992 film Bram Stoker's Dracula suggests that Renfield (portrayed by Tom Waits) was Harker's predecessor as Count Dracula's agent in London; it is implied that this is the reason for his present madness.
Renfield appears as the protagonist in a number of works that provide his backstory or retell the story from his viewpoint. The novels The Book of Renfield by Tim Lucas and Renfield: Slave of Dracula by Barbara Hambly are examples of this, as is Gary Reed's graphic novel Renfield: A Tale of Madness.
In Jim Butcher's Dresden Files series, a "Renfield" is a term for a Black Court thrall that has been "psychically sandblasted" into submission to the vampire. They are incredibly strong, psychotically violent, and have a tendency to "self-destruct" into violent mania after one or two years of thralldom. Dresden describes one as being "less than an animal".
- Dracula. SparkNotes; Character list.