The narrator is a participant, first-person narrator and he is directly involved in the story. Because he is ever-present, we get a true insider's view of what is going on with the other two characters, Madeline and Roderick. Through the narrator's thoughts, we are able to ascertain what type of person he is. He is very educated and intelligent and is able to think critically and analytically about the situation at hand and about Roderick.
The narrator appears to be very reliable and believable due to his rational, logical demeanor and ability to discuss what is going on in a rational manner. One could always argue, though, that after Roderick encounters Madeline (the narrator does, too) that the narrator perhaps might have lost his grip on reality (as many of us might).