M. Caldwell is a secretary at the New York Bulletin who was a mole for Wilson Fisk.
Working for Wilson Fisk
While working for the New York Bulletin, Caldwell was approached by Crime Kingpin Wilson Fisk and ordered to use her position at the Bulletin to send him any information regarding the press' knowledge of his criminal activities.
Hell's Kitchen Burns
Mitchell Ellison called all the staff at the Bulletin together when reports came in of massive explosions all across Hell's Kitchen, which he blamed on a possible gas leak. Ben Urich asked to know the locations of the explosion and Ellison ordered Caldwell to give him the list, as he read out the locations Urich immediately ran out of the building, claiming that the building that were attacked were owned by the Russian Mob, therefore this was a bombing attack.
The Firing of Ben Urich
Caldwell was present went Ben Urich spoke to Mitchell Ellison and demanded that he print his expose on Wilson Fisk, which Ellison refused as he claimed it was not backed up and would result in them being sued. Urich then accused Ellison of being the mole within the New York Bulletin, not realising that it was in fact Caldwell; in response Ellison fired Urich on the spot. Caldwell informed Wilson Fisk of the events and how Urich would likely print the expose online. That same night Fisk broke into Urich's home and murdered him.
Wilson Fisk's criminal empire was exposed when Carl Hoffman went to the FBI to confess his crimes and his involvement in Fisk's crimes. As a result everyone connected with Fisk's empire was arrested by the FBI, including Caldwell, who was pulled away from her desk while Ellison and the rest of the staff could only watch in horror.