The two most important main characters at the outset of the story are Jack and Terese. Laurie enters in as a significant character later on as does Richard.
Jack is the protagonist who used to be an ophthalmologist but is now a forensic medical examiner who performs autopsies and makes diagnoses of cause of death at the city morgue. Jack's career backstory is that his ophthalmology practice was taken over by the medical corporation AmeriCare so he was forced to retrain. His personal backstory is that his wife and child were killed in a car accident five years before the story opens.
These backstory elements are important to character description because they incite character traits and provide the source of both external and internal character conflict of the Person against Society and of the Person against Self sorts. These slightly artificial and contrived conflicts are the reason Jack has the character traits he does and takes the actions he does.
Jack is reckless and careless of his safety though deep down--under the bitterness, rage, self-reproach and cynicism that has developed since his losses of family and career--Jack is diligent, dedicated, motivated, caring, compassionate, sensible and reasonable. When the novel concludes, we realize Laurie saw these deep down qualities just as we did. As a side note, Cook can be congratulated on carrying us along with Jack and simultaneously revealing both sides of his present psychological duality as the mystery unravels.
At Thirteenth Street Jack turned east, crossed First Avenue, and abruptly turned [his bike] into the loading bay of the Office of the Chief Medical Examiner for the City of New York. ... Jack wheeled his bike past the security office and waved at the uniformed guard.
Terese is a hard driving, pushy, inconsiderate, compassionless, self-centered advertising "creative" in charge of the department that develops the artistic creative side of advertising campaigns. This "creative department" distinction is important to one of the subplots to the story in which two advertising account representatives (Helen and Robert) become highly competitive with Terese, even sabotaging her efforts, because Terese and an account representative (Robert) both want the same appointment to agency president. It turns out that Terese is also conniving and manipulative with a low (or absent) sense of moral probity and ethical good. As the story concludes, we are glad we never really liked her.
Laurie, who is important later, is very much like Jack's better self: she is loving, competent, diligent, compassionate, caring, sincere and trustworthy. We like Laurie and in the end are glad we do.