The Assassin (name is never given) is one of the main antagonists of the Kingdom of Liars series. She was hired by an unknown person to kill Sir Brandt, and she, in turn, hired the Fithling Scientist to design a weapon for her that would be undetected by the Elk Guard's magic. She murdered Brandt's brother, thinking it was Brandt himself, along with everyone aboard the train except for the hero and his sister.
She then attempted to tie up loose ends by killing the Fithling Scientist, but he had locked himself up inside his warehouse, leaving his guards to suffer her wrath. Finally, she made her way into city hall via the sewers, and disguised herself as a waitress. Whelek was alerted the assassin was already in City Hall, and had the Elk Guard on alert. When the hero confronted her, she revealed her custom-made weapons concealed within her. She aimed for Sir Brandt, but Vic'eren took the shot instead, and the assassin was apprehended.
It was never revealed who had hired her to begin with, but the hero theorizes the possibility of it being Sir Brandt himself in order to get publicity. It is also interesting to note that there are some similarities between her and Mary Ann, as well as Alice (primarily their facial characteristics and chosen profession).