Much of the novel is from Dick’s perspective, and many critics will say that the novel chronicles his decline from brilliant psychiatrist to drunken bum. If the novel is about how "smart men go to pieces," as some of the characters discuss, then he’s the most obvious choice for protagonist.
Much attention is given to Nicole in the novel, as she is a complicated character who has suffered traumatic life experiences. She is also the only character who gets to narrate a section of her own. Aside from than that, Dick and Nicole probably get about equal airtime in the novel.