Initially portrayed as jocular and immature, such as having a bulletproof vest made up for himself that says "WRITER" rather than "POLICE", Castle's character deepens as the series progresses. In "Sucker Punch", while attempting to catch the killer responsible for the death of Beckett's mother over a decade ago, Castle willingly donates $100,000 of his own money for a fake hit, to lure out the killer. He then offers to terminate his partnership with Beckett after she was forced to shoot the real killer, revealed to be a contract assassin silent about his employer. Beckett rejects the offer on the grounds that Castle makes her hard job more fun.