Madison Beaumont (Natasha Hall) was a villainess from "Smells Like Teen Spirit," episode 6.15 of Castle (airdate February 17, 2014). She was a student at Faircroft Preparatory Academy, a former alma mater of Richard Castle.
As revealed later on, Madison was the ringleader of a Bling Ring-type trio; she and her best friends, Kris Howard and Hillary Cooper, would break into the homes of their also wealthy Faircroft schoolmates and rob them. The villainess upped her thieving game after learning about a former Triad member named Chen who was accused of stealing millions in bearer bonds during a bank robbery from her father, Charles Beaumont, who was his attorney. Madison went on her own and stole the bonds from Chen, and she hid her ill-gotten loot in a hollowed out copy of War and Peace in the school's library. The villainess used her robberies to secretly pay her credit card bills, as she was a shopaholic and wanted to hide her exorbitant spending from her parents.
One month prior to the episode, when school resumed after winter break, Madison got caught in the library early in the morning by her classmate Jordan Gibbs near the hiding place for her loot. While Jordan didn't see what she had really been doing, Madison feared that Jordan could expose her and started bullying her along with Kris and Hillary to make her too scared of them to speak out.
As part of her schemes, the evil Madison colluded with another student, Lucas Troy, to scare Chen out of his home so she could steal the bonds. She had been blackmailing Lucas after learning that he used his apparent special effects talent to scare her and her friends into backing off of Jordan by convincing them that she had supernatural powers. Madison also forced him to use his talents to help her prank Kris, who was especially rattled by the incident with Jordan and wanted out of the ring, by rigging up a telekinetic event in her room. The video made it look like Jordan's "telekinesis" killed Madison by throwing her into the ceiling. After Madison responded to Kris' pleas by threatening to ruin both her and Hillary, Kris decided to end her friend's threats by killing her, and used the prank video as an alibi to cover up the murder. Madison's reveal as a thief came posthumously after Castle and Kate Beckett found the stolen bonds inside the library book.