In a city of largely invisible staff workers, Marine Corps Lt. Col. Oliver L. North of the National Security Council staff has emerged as an influential and occasionally controversial character in the implementation of the Reagan administration's foreign policy.

North, a trusted aide to national security affairs adviser Robert C. McFarlane, has by most accounts been obedient and creative in his work, which has thrust him into a pivotal role in the administration's Central American policy -- particularly in nurturing connections among antigovernment rebels in Nicaragua, their conservative supporters in this country and the rest of the administration.

But his work on behalf of McFarlane has taken North into other areas as well, testimony in part to his diligence and high energy. North was involved in drafting a letter from President Reagan to Syrian President Hafez Assad that reportedly angered Assad and irritated State Department officials who tried to have it altered.

He is little known to the public, but to those in the administration and among those actively involved in the debate over Central American policy, North has been a visible player.

In that capacity, his activities raise questions about a gray area of government policy. At a time when Congress had voted to outlaw direct aid to the anti-Sandinista rebels in Nicaragua, North worked actively to assure the counterrevolutionaries, or contras, that the U.S. government fully supported their cause.

Administration officials insist that North stayed within the letter and spirit of the October 1984 law, which ended three years of U.S. intelligence community help for the contras. The administration had fought the ban, arguing that the rebels, which it calls "freedom fighters," were key to pressuring the leftist Sandinista government of Nicaragua away from its Soviet supporters.

But Congress acted out of concern that the insurgents had been using secret U.S. funds illegally in trying to overthrow the Sandinista government, and that contra methods had been brutal and immoral. The checks that went out in May were the last of an estimated $80 million in U.S. aid.

McFarlane said in an interview he had told his staff to comply with the law. "However, we had a national interest in keeping in touch with what was going on, and, second, in not breaking faith with the freedom fighters," he said.

"What does it mean not to break faith? Nothing more or less than making it clear that the United States believes in what they are doing," McFarlane continued. "We could not provide any support, but we made it clear we would continue seeking that support" from Congress.

"We wanted to give a continuity of policy. This could not involve money or material support. It was a matter of hand-holding," he added.

North, 42, he indicated, was put in charge of holding hands.

In part as a result of North's efforts, Congress last month approved $27 million in nonmilitary aid to the rebels, ending a virtual seven-month halt in their activity.

Interviews with dozens of people who know North found general agreement that his power is the kind that comes from being the center of an information network, trusted by superiors and contacts to interpret his knowledge for their mutual benefit. While his recommendations have sometimes been rejected, they have always figured heavily in policy debates.

If McFarlane is Reagan's senior national security adviser, North is "McFarlane's McFarlane," one senior government official said, "except that he's much more activist than McFarlane."

North said he was willing to be interviewed, but McFarlane refused. "He's not a rogue elephant" but rather "like a son of mine," McFarlane said.

In an article in Friday's editions, The Washington Post withheld North's name at the request of White House spokesman Larry Speakes, who said North was concerned about his safety. North's name has been used by other news organizations, and White House officials were informed on Friday that The Post intended to use North's name in this article unless there was a compelling reason not to do so.

"I personally cannot tell you that the other side has taken terrorist actions againt Americans in positions of responsibility ," McFarlane said. His preference to keep North's name out of the paper was "more a matter of my integrity," McFarlane said, adding that a career officer "acts at someone else's direction" and that the principal, rather than the staff member, should shoulder public responsibility.

A San Antonio native and graduate of the U.S. Naval Academy, North was "a platoon and company commander in combat and participated in both conventional and unconventional warfare operations in Southeast Asia" as a U.S. Marine during the Vietnam war, according to his official 1983 biography.

He was awarded a Silver Star and two Purple Hearts, among other decorations, and has taught since at the Marine Corps Basic School, the FBI Academy and a special operations school on Okinawa, returning to Washington in 1975 to be plans and policy analyst at Marine headquarters.

North is reported to have worked with McFarlane, a former Marine from Texas, at the Pentagon on Middle East affairs and was sent to the NSC in August 1981. He is now deputy director for political-military affairs, focusing on Central and South America, and is "responsible for national-level contingency planning, crisis management and counterterrorism," the biography said.

North "has appeared as a guest on William F. Buckley's 'Firing Line' " interview program, the biography noted.

In a divided, sometimes chaotic and turf-conscious NSC staff, North stands out as a hard-working, articulate and efficient facilitator, according to most of those interviewed.

At the State Department, North is suspected of responsibility for wording in a July letter from Reagan to Syrian President Assad that reportedly angered Assad just as the administration was hoping for his help in obtaining the release of seven Americans who remained hostage in Lebanon after 39 others from TWA Flight 847 were released. North "didn't accept the changes" in the wording that the State Department wanted to make, one department source said.

McFarlane said the letter was developed by the "terrorist incident working group," which includes people from several agencies, and that North "absolutely did not drop out" and "did not ignore, discount or overrule" anything from the State Department. Administration officials stressed that North was not the sole author of the letter.

In 1983, North was awarded the Defense Meritorious Service Medal "in recognition of his contributions in securing approval" from Congress for the "air defense enhancement package for Saudi Arabia," or the AWACS airborne warning and control system plane.

An intelligence official said North drafted a directive on terrorism that liberally used the word "neutralize," meaning kill, and that the draft was toned down before Reagan signed it.

Other officials said North was the "nerve center" for planning the 1983 U.S. invasion of Grenada, one of very few people who knew everything going on, and that he played a role in the decision to send Vice President Bush to El Salvador in December 1983 to warn leaders there that continued death squad activity would halt U.S. support.

Leading conservatives cited North as their principal administration source of information on the Nicaraguan conflict and said he has been directly involved in shaping administration domestic political strategy on the issue.

North was a mainstay of the White House Office of Public Liaison's "outreach project" on Central America, which was intended to muster public support for administration views. With his maps, diagrams and charts and colorful delivery, North "became the briefer of choice" for the mostly conservative groups that attended, said Morton Blackwell, a conservative activist who was then a presidential assistant in charge of the project.

In weekly planning meetings for the project among the NSC, the Central Intelligence Agency, the State Department and the U.S. Information Agency, North emerged as one of the staunchest and most forthright conservatives, "one of the best," Blackwell said. "He's one of us."

McFarlane said North "has probably briefed 100 groups," always at McFarlane's direction. Those who attended say the briefings typically deal with battlefield conditions and prospects for contra military advances.

North has been particularly close to the leadership of Citizens for America (CFA), led by the flamboyant tycoon Lewis Lehrman, who ran for governor of New York in 1982. Sources close to the group said North was in almost daily touch with former CFA executive director Jack Abramoff, and helped select contras for a speaking tour of the United States that CFA organized during the key April congressional debates.

"He's in close touch with most of the New Right people," said Rep. Vin Weber (R-Minn.), a leader of the House Conservative Opportunity Society. "He's pretty highly regarded as one of the better guys on the inside, pretty hard-line."

When conservatives asked after the funding cutoff how they could help the contras financially, North would respond that he "can't tell them what to do" and that the contras "are in the phone book," McFarlane said.

Retired Brig. Gen. Harry C. Aderholt, president of the Air Commando Association, which claims to have distributed more than $30 million in donated medicine to hospitals in Honduras and El Salvador, said he "used to talk on the phone to Ollie all the time about El Salvador" early last year.

Although Aderholt denied sending any aid to the contras, his group's July 1984 newsletter reported that "shipment three" of medicine intended for El Salvador "was diverted to Honduras because of the cutoff of funds to the program in that area. This was requested by certain high-level people . . . . "

"Ollie knows where we stand, and he's for us 100 percent," Aderholt said.