Margaret Hooper was the Senior Assistant to the White House Chief of Staff.
Margaret was generally rather eccentric and exhibited a number of personality quirks. She once revealed to Leo that she had practiced the President's signature "just for fun", much to Leo's dismay. Despite her sometimes quirky behavior, Margaret was highly competent at her job and exhibited unconditional loyalty to Leo. She was a mine of information, a fact acknowledged by Ginger to Sam Seaborn in "20 Hours in America (Part I)" -
Sam: "Do we have some sort of condensed... Reader's Digest index... of, well... all human knowledge?"
Ginger: "We usually just use Margaret."
As with the President and Mrs. Landingham, Margaret had a complex, almost symbiotic relationship with Leo, although in their case it was essentially more of a father-daughter dynamic. On one occasion, for example, when Leo was interviewing Ainsley Hayes for a position in the White House Counsel's office, Ainsley remarked that Margaret seemed to be a very good secretary. Leo observed that she would be glad to hear that, given that she was listening on the other side of the door, and then proceeded to bang a fist on the door, drawing an 'Ow!'.
After she was promoted to Chief of Staff, C.J. commented that she found the fact that Margaret was also tall to be "reassuring". This was beneficial once when C.J. had to borrow a top from Margaret after having been drenched in coffee.
She became pregnant, and once mentioned that she had relatives in Canada.
During the investigation of the White House Leak of the secret military space shuttle, Margaret was subpoenaed. Senator Dresden focused his questioning about C.J. Cregg's phone calls with Greg Brock and her ability to listen in on those calls. The proceedings lasted for an entire day. After she returned to the White House, Toby Ziegler confessed to being the leak and C.J. had her call the White House Counsel's office to send someone over. (Mr. Frost, Here Today)
In the season six episode "The Wake Up Call", it is implied that Margaret might be a lesbian. Toby is talking to a beauty-pageant winner, when a series of male White House staffers, along with Margaret, interrupt to get a look at her.
In a separate incident, two episodes later in "Drought Conditions", Josh comments upon noticing Margaret's pregnancy that he "didn't even know she was married", after which he is given a significant glance by C.J. indicating that he had forgotten something important, while C.J. is out of focus in the shot. It might be presumed that Margaret is a lesbian and is still unable to get legally married. Josh simply says "Oh, right" and moves on.