Similarities to Richard Castle
As with Castle, he has a playful sense of humor, gets himself in trouble when he fails to heed Nikki's orders and has an ex-actress and diva for a mother, who he has nicknamed The Grand Damn. And as some have noted, the chess piece called the rook is usually shaped liked a castle.
Differences from Richard Castle
Unlike Castle, he has no children and is a world-renowned journalist, writing for the magazine First Press.
Relationship with Nikki Heat
He meets Nikki when he is assigned to write a magazine article on New York City police officers and detectives and is assigned to shadow Nikki for research. As he helps Nikki some of the murder cases she is assigned, they find themselves very attracted to each other and, in Castle's first Nikki Heat book, Heat Wave, they begin a relationship. It is revealed in Naked Heat, the second Nikki Heat book, that once Rook's article on Nikki and her squad was published, their relationship soured when Nikki found that her sudden fame made it difficult to do her job. She also took umbrage to Rook's making her the center of the article, minimizing the involvement and importance of her squad members Detective Ochoa and Detective Raley to her ability to solve cases.
In Naked Heat, he reveals to Nikki that, in addition to writing for First Press, he also writes romance novels under the pen name Victoria St. Claire.