Narrator is the person (perspective) which is chosen by the author to tell the story (literary work, movie, play, verbal account, etc.) to the readers (audiences). Traditionally, the narrator is supposed to be reliable, since he/she/it is the only connection between the readers and the fiction world. But occasionally, authors would use unreliable narrator to be the perspective of their story.
The concept of the unreliable narrator (as opposed to "author") became more important with the rise of the 18th Century. Until the late 1800s, literary criticism as an academic exercise dealt solely with poetry (including epic poems like the Iliad and Paradise Lost, and poetic drama like Shakespeare). Most poems did not have a narrator distinct from the author. But novels, with their immersive fictional worlds, created a problem, especially when the narrator's views differed significantly from that of the author.(Wikipedia)
Unreliable narrator is usually adopted when the author try to create suspense to the readers, make them more critical suspicious and avoiding passive reading. To let them think in different perspectives and reconstruct the truth through their own cognitive thinking. It involves a more interactive process then the traditional reliable narrator, the readers just need to sit tight and receive everything from the narrator. Unreliable narrators can be classified into two main categories, those cannot be fully trusted because they do not understand what they are narrating (Robert Walton) or those who are simply lying to the readers to suit their needs or justify their faults (Victor Frankenstein). In Frankenstein, Mary Shelley used unreliable narrators to tell the story, to make the readers question the truth told by different narrators and created a huge room of imagination to them.
There are two narrator in Frankenstein were considered as unreliable, Robert Walton, an Arctic seafarer whose narrated his part with the letters for his sister in the beginning and the end of the story; Victor Frankenstein, a talented scientist created a monster whose narrated most portion of the story.
Robert Walton, who captained a North Pole bound ship, was a very good friend of Victor Frankenstein. Saying a good would be too neutral, the relationship between them would be more accurately to be admiration. "For my own part, I begin to love him as a brother; and his constant and deep grief fills me with sympathy and compassion. He must have been a noble creature in his better days, being even now in wreck so attractive and amiable." (Shelly, Frankenstein) that was the description of Robert Walton's opinion towards Victor Frankenstein; we can see how obsessed Walton was.
Considering the bias of Robert Walton, his narration became unreliable. He trusted everything Victor said without any judgment. His determination was unreliable, and therefore, he became a reliable narrator.
Actually, Robert Walton's situation of unreliable narration was very much similar to the movie "The Sixth sense". In "The Sixth sense", Cole Sear, a troubled boy who claimed to see ghosts who don't know they're dead, seek for help of a child psychiatrist, Dr. Malcolm Crowe. Because of this, he was called a freak in school. Crowe, at first thought he was seeing things, but after spending a lot of time with Cole, he discovered Cole may be seeing dead people after all. At the end, Cole discovered that Dr. Malcolm Crowe was actually a ghost, who didn't realize his own death.
Comparing Robert Walton with Cole Sear, they are both defiantly unreliable narrators. But they were not intended to trick the readers/audiences; they were both misled by others. In "Frankenstein"; In "The Sixth sense" the person who misled Cole Sear was his psychiatrist, Dr. Malcolm Crowe. They both trusted the misleader with their heart, but turning to be unreliable narrators following their wrong judgment.
In contrast, Cole Sear would be a "better" unreliable...