For five weeks, Iran-contra congressional investigators have been learning more and more about the extraordinary activities of the man who isn't there. The more they know, the more critical and complicated the role of Oliver L. North becomes -- and the more questions about him arise.

Lt. Col. North's presence at the hearings has become so pervasive that he now diverts attention from President Reagan. Questions initially asked about Reagan now are raised almost exclusively about North.

"We don't know what he knew when he knew it," said Rep. Michael DeWine (R-Ohio), who began the hearings as one of North's strong defenders. ". . . There's a lot of things we don't know, and so I think we have to reserve final judgment in regard to him."

They'll be learning more about North this week when his former secretary, Fawn Hall, is to testify about her involvement in helping him alter and destroy White House documents.

The portrait of North that has emerged from testimony is of a man both admired -- even loved, as businessman Albert A. Hakim said repeatedly last week -- and feared.

North has been established as the person who knows the most about what really happened in the Iran-contra affair -- and under what authority. His testimony could save -- or destroy -- many of those involved.

Witnesses such as Assistant Secretary of State Elliott Abrams, the president's "point man" for Central American policy, who were quick to denounce other officials involved in the affair, have been notably reticent to criticize the Marine lieutenant colonel who was fired from his National Security Council staff position last November after his role in diverting U.S.-Iranian arms sale profits to aid the Nicaraguan contras became known.

However else history judges North, he is likely to go down as one of the, if not the, most remarkable secret agents America has known, and almost certainly as one of the most important witnesses in presidential history.

Only months ago hardly anyone had heard of him. Now say "Ollie" and almost everyone, everywhere, instantly recognizes the reference to North. He has entered the American folk vocabulary. To his admirers, he fills a void in the hero-less America of the '80s: He is the John Wayne of the times. To his detractors, he is a consummate con man, the worst of the Cold War types and a man whose secret operations skirted if not broke the law and produced the most serious constitutional confrontation between the White House and Congress since Watergate.

The range of his activities now disclosed is staggering. North seems to have been at times acting president and prime minister without portfolio. He was the commander in chief of the secret army, air force, even navy, formed to swap arms to Iran for hostages and provide covert support to the contras. Testimony thus far shows he was also fund-raiser, speechmaker, political strategist, negotiator, secret White House tour guide for visiting Iranian officials and tireless operative who hardly, it seems, even had time to eat or sleep given his mulitiple activities and clandestine overseas trips.

And, of course, he was Ronald Reagan's "national hero," to use the president's phrase. Yet, as the testimony suggests, North was also betrayed or a betrayer, or both. He seems destined to end up as the Iran-contra scandal's dupe or villain, or both.

As all this often contradictory detail has poured forth, committee members have been forced to reexamine their impressions of North, especially after last week's disclosures that attempts were made to funnel secret funds from Swiss banks to North's family.

One key member, who has been examining the record on North for many weeks, said: "He comes over as a far more complex person than the picture of him drawn, for example, in the Tower commission report. The picture of him then, produced by his own private words from those NSC Profs {National Security Council classified computer messages}, is of a relatively simple gung-ho guy. It's like the Nixon tapes: Once those became public, you had the real portrait of Nixon. That's not so with North. He is much more complex, much more manipulative, much more imaginative and, I would say, a driven and devious person."

To Sen. Warren B. Rudman (R-N.H.), vice chairman of the Senate select committee, North seems to be a zealot, the kind of man Rudman would have liked serving with him during combat in Korea but not running foreign policy covertly out of the White House.

To Rep. Ed Jenkins (D-Ga.), on the House select committee, North is either a "sincere zealot" or something more troubling. Jenkins, like other members, finds testimony about Iran-contra money set aside for North and his family deeply disturbing.

To Sen. Daniel K. Inouye (D-Hawaii), Senate committee chairman, North is a man who has wielded almost unbelievable power, "who everyone suggests is second only to the president of the United States."

To Senate chief counsel Arthur Liman, North seems to have been part hustler as well as devoted patriot. An exchange Thursday between Liman and Hakim, the money man of the secret contra-resupply operation, was revealing:

Liman: "Didn't North invoke the name of the president?"

Hakim: "Yes."

Liman: "He dropped the name of the president all the time, didn't he?"

Hakim: "When I was present."

Liman: "Did you consider him to be a name dropper? That it was puffing?"

Hakim: "I found Col. North to be a devoted American. He will do anything to obtain his objective, sir, including giving his life."

Liman: "I have a question about this. I mean, I -- you can get an impression here that North is hustling you, that you're hustling North, that the Iranians are hustling you, that you're hustling the Iranians and it really is, as you said before, you know, a commercial type of environment. Is that an unfair impression that I have?"

Hakim thought it was unfair. His deposition before the committee, given two weeks ago, was also revealing about the range of North's activities and his influence:

When asked whether North "could, in effect, call on the enterprise for money for whatever purpose he might have," Hakim answered, yes, then added:

"Richard said that you never know what Ollie would need next." Hakim was referring to his partner, retired Air Force major general Richard V. Secord.

Hakim's deposition continued. Question: "I take it at one point Ollie needed a ship and you used the money to buy a ship?"

A -- "Yes."

Q -- "And at one point Ollie needed Motorola radios and you used the money to buy Motorola radios."

A -- "Yes."

Q -- "And I take it at one point he wanted some money for DEA {Drug Enforcement Administration} agents in cash and you used it for that purpose."

A -- "Well, I don't know if he initiated that. Richard told me that such and such people will come to you and give him so much."

Q -- "But it wouldn't come as a surprise to you to learn that also was originated by Ollie North."

A -- "I would be surprised if it didn't."

Q -- "Is it fair to say that it was your understanding that during that period of time that this differential surplus, profit, whatever we call it, was being used and was to be used at North's direction?"

A -- " . . . Yes."

Hakim recalled that one day Secord said: "Who knows, if we do a good job, the president may send us to Angola."

"In my mind," Hakim added, "I said, 'You know, the way this crazy thing is going, they might do that.' " They joked about North wanting to go to Angola, and actually established a business category to cover Africa in case that occurred.

There was no humor for North in last week's disclosures about Hakim's efforts to get money to his wife.

Rudman posed the problem for North this way when he questioned Hakim:

"I want to put this question to you to ponder because we're going to have to hear from Col. North," the senator said Friday.

"Certainly, Mr. Hakim, you are sophisticated enough to know that by asking Col. North's wife to come to Philadelphia -- and I don't know if that happened or not; we'll find out -- by trying to divert money through real estate firms for services maybe not rendered in full, by naming him in a will and by doing other things that were done, Col. North was going to be put in tremendous jeopardy, because let us go on the assumption that Col. North never asked for any of this to be done. Col. North is now in the position of not having informed the United States' authorities that these attempts were being made. Don't you -- did you not realize at the time, with Mr. Zucker {a lawyer and Hakim business associate}, who understands U.S. laws and ethics, that even a good faith . . . effort to help Col. North would put him in tremendous jeopardy?"

Hakim testified he regretted not having anticipated that kind of a problem for North. That statement led Rudman to comment:

"One of the unintended -- and I emphasize unintended -- effects of this getting private businessmen involved with U.S. officials and U.S. military is the possibility of very corrupting and very corrosive effects on our government."

The committees have a long way to go in determining whether the Iran-contra affair was a well-intended scheme gone catastrophically awry or a case of classic corruption, or perhaps both. But they already know one fact. At the center of the riddle stands the figure of Oliver L. North.