In her own self-portrait, Fawn Hall was the quintessentially loyal secretary -- and something more.

She was, as her testimony yesterday made clear, so totally dedicated to the cause exemplified by her boss, Lt. Col. Oliver L. North, that she was willing to do almost anything to help him. And that included misleading officials; destroying, altering and absconding with incriminating official documents and placing her career in legal jeopardy.

The strain was visible in her two hours at the witness table yesterday afternoon in the Rayburn House Office Building. She was trembling when she raised her hand to take the oath before beginning her long-awaited testimony shortly after 3 p.m., and her face was lined with fatigue when she ended. But throughout she proved to be a determined witness with a clearly defined mission and rationale of her own.

By her own account, Fawn Hall thought of herself as a player -- indeed, she pursued a policy of her own -- no less than North and the others who became involved in the Iran-contra affair.

"It was a policy of mine not to ask questions," she said briskly, when pressed about a number of crucial Iran-contra events in which she was a witness or a participant as North's personal secretary.

She felt "a little uneasiness when he asked me to do it," she said, describing North's instructions to alter documents relating to the diversion of profits from Iran arms sales to aid the Nicaraguan contras, "but I believed in Col. North and . . . I did what I was told."

At another point, she said:

"I had no right to question him {North} . . . . It was my policy not to expose or talk about what I was doing."

Her testimony, delivered in brisk, almost staccato manner, with many curt "yes" and "no" responses, was marked by lapses of memory and admitted lack of curiosity about what was happening around her. But it provided the strongest, most dramatic documentation of a cover-up in the Iran-contra affair yet made public.

At the heart of all her actions was her deep commitment to North personally and to what he represented in terms of policy.

"For the nearly four years that I worked for Lt. Col. North," she told the committees in a prepared opening statement, "my hours were long and arduous but I found my job to be most fulfilling. During my tenure at the National Security Council, I came to have enormous admiration and respect not only for Lt. Col. North but for many with whom we worked. I admired them not only for their hard work and professional abilities, but also for their selfless and deep sense of dedication and loyalty to the president and our country."

She praised North for "his ability to be a friend when one is needed" and twice spoke with affection of North's wife, Betsy, and their family.

In the end, though, Hall gave testimony that was devastating to her former boss -- a fact that seemed to trouble her as she spoke. At times she seemed to be apologizing to North directly.

But she had accepted a grant of immunity that forced her to tell the painful story of how she tried to protect North by covering up what she had seen and done on his behalf. That included running an NSC shredding machine until it jammed and broke down, adding North's initials to altered documents and following his instructions to make deletions and substantive changes in key documents.

She also told of trying to console North by assuring him it couldn't be true that he had been fired by President Reagan last Nov. 25, and of crying when she realized he had indeed lost his job.

Before her long-awaited appearance yesterday, Hall had denounced as "sexist" the comments of a committee member, Sen. Howell Heflin (D-Ala.), who said he believed that she had removed classified documents from the White House complex by stuffing them inside her clothing and brassiere.

That wasn't the way she did it. "I probably took the altered copies . . . folded them and placed them inside my boots," she said yesterday, "and put them . . . in my back."

She reached behind and patted her waist, indicating she had slipped them inside her undergarments.

But typically, Fawn Hall didn't take that action alone. Loyal to the end, she first checked with North. She asked "if he could see anything in my back."

When North assured her that nothing was visible, they left the Executive Office Building together.