The assassin is a hired killer on Mortimer's payroll, responsible for protecting the interests of the scheming front desk manager, who in private, intends to return the White Glove Society to cannibalism. He and a few White Glove supporters of Mortimer are under orders from him to stop anyone who is aware of or is in opposition of his goals. One of these people, an insider named Chauncey, along with his detective contact, Jay Barnes (or Crusoe), are targets of Mortimer's associates.
The assassin can be seen in a small room below the steam room. He can be seen in this room by using the tclconsole command for a closer look. He can also be found on the Xbox by going through a wall which is to your right after you enter the room, after you cross the corner go straight and you will end up a shady area. You will see a room for a short while because you fall and end back up in the place you started. It is possible to get there too, by somehow making a leap of faith. If you talk to the assassin you will only "look" at him for a split second then going back to normal. This is because the assassin uses an empty voice type. If you kill him, Chauncey also dies.