Numbuh 501, real name Mary D. Luis, was, at a time, the Head of Decommissioning. She appears to be from England and has a British accent. Her first appearance was in "Crutch" where she over saw Numbuh 414's decommissioning.
In the story "Agenda", she is the main villain. She at first declines Numbuh 362's request for back up. She later on knocks out the Members of Sector Q. Her main goal in the story is to aquire Numbuh 74.239's new invention, the Teen Locator, in better hopes to skip out on her birthday and to recruit new teens into the Teen organization.
She personally saw to Numbuh 362, Sector Q, and their informant Mario's capture. She then revealed her plan and motives to Rachel and the departed to the science fair. She later shot Numbuh 1 in the back and fought Rachel. She was later seen at the end of the battle being taken away for decommissioning.
Mary reappears in the story "Ethics", she is revealed to be not decommissioned but infact imprisoned in the KND Arctic Prison. She is also responsible for Figure's decommissioning and his descent to insanity. Near the end of "Ethics", Mary is decommissioned by Figure.
In the KND: Universe, most of Mary's backstory is still the same. The events of 'Ethics', however' never took place. Mary is set to return, having escaped the Arctic Prison, under the name 'Bloody Mary' and being one of Sandman's Rogues Gallery.