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.
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 has 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. Then, 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, presumably that Margaret is a lesbian. Josh simply says "Oh, right" and moves on.