Chief Bromden is a Columbia Indian who suffers from schizophrenia. Although he plays a central role in the story, he is largely an observer. Chief is an interesting narrator because he is certainly not unbiased, and his mental illness can also shed doubt on his reliability. Chief goes in and out of feeling that he’s in a fog, has numerous hallucinations, and believes an elaborate conspiracy theory about the world being a machine called the Combine.
In the first chapter, even Chief himself brings up the issue of his reliability. He says, "God; you think this is too horrible to have really happened, this is too awful to be the truth! But, please. It’s hard for me to have a clear mind thinking on it. But it’s the truth even if it didn’t happen."
Essentially, Chief does have a mental illness and he doesn’t have a "clear mind," but does that mean he’s not a trustworthy narrator? Chief insinuates that at least some of the story he’s telling didn’t actually happen—but he says it's stilltrue. It’s up to the reader to decide, in this novel, whether or not to trust Chief.