In 1961, when Gen. Park Chung Hee moved tanks and troops over the Han River to overthrow a hapless civilian government here, the architect of his coup was a young, out-of-uniform lieutenant colonel named Kim Jong Pil.

After more than 18 years, many of them spent at Park's side, Kim is emerging as the most likely choice of Park's heirs to succeed the slain president . If the plan approved by most influential South Koreans is followed, elections will be held in about one year to replace a caretaker president. Kim is already the odds-on choice for the nomination by progovernment politicians.

If he succeeds, it would be the culmination of a topsy-turvy career. Often he was Park's closest confidant, the designer not only of Park's coup but of political institutions that served him. At least twice, he mysteriously fell from grace, seemingly banned from Park's presence.

To many South Koreans, Kim was the strong right arm that hammered out the underpinnings of Park's rule. He founded, in 1961, the Korean Central Intelligence Agency, which became the widely feared apparatus that spied on and allegedly tortured the president's enemies. He founded the Democratic Republican Party, which provided a semblance of parliamentary democracy in the legislature when all power actually flowed from the president.

Others believe that he frequently tried to soften Park's rule without opposing it publicly and tried unsuccessfully to transform the National Assembly into a place of influence and free debate.

One opposition member of the assembly who is bitterly critical of the government regards him as a "reasonable, mature" man who tried to encourage debate by befriending critics with small holiday gifts and invitations to private conversations. Kim strikes another source as a "political animal" who understands and approves the give-and-take of democratic politics.

The Kim who has emerged from Park's shadow since the Oct. 26 assassination is a determined peacemaker who speaks of a new era of "humanitarianism, liberalization and diversification." Elected partly chairman overwhelmingly, he moved promptly to mend fences with the opposition New Democratic Party that Park had intended to destroy.

In an unusual move, he met last week with Kim Young Sam, the New Democrats' president who was expelled from the assembly; agreed to hold more dialogues, and announced that both parties would share seats on a committee to revise Park's constitution. Asked what should happen to the assembly's Park-appointed speaker -- a man widely hated by the opposition -- kim said he would help the man into an "honorable retirement."

The moves are taken as evidence that Kim wants to broaden his base of support outside the ranks of Park's followers, apparently in preparation for an open election late next year, if it takes place. Many observers believe that could become a contest between him and the former opposition candidate, Kim Dae Jung, who almost beat Park and who is under house arrest here for criticizing the government before Park's assassination.

Kim Jong Pil, 53 is described by acquaintances as a reserved man with a wry sense of humor, different from the hearty back-slappers commonly found in South Korean politics.

He is frequently "melancholy," according to one legislator, who said Kim rarely exposed his private thoughts. "I get the impression of a complicated, fairly intricate man who has moved very carefully all his life," another acquaintance said.

He is an amateur musician and an accomplished painter who has published a book of his own paintings. His family home is in a central South Korean region known for its scholars. An articulate public speaker, he is said to have a rare ability to move large audiences with oratory.

A graduate of the South Korea Military Academy, Kim was a young lieutenant colonel who apparently resigned in disgust with the post-Korean War government of President Syngman Rhee prior to helping Park plan the 1961 coup.He promptly became jack-of-all-trades for the government. As a diplomat, he led negotiations that brought normal relations with Japan.

He was the first director of the KCIA, whose initial mission was ferriting out North Korean Communist spies and which subsequently turned its surveillance operations against any dissenter, here and abroad.

There have been published reports that Kim also was instrumental in forming the Rev. Sun Myung Moon's Unification Church, an organization that is controversial in the United States for its enrollment of American youth and alleged brainwashing activities.

In a report published last year by the House International Relations Committee, U.S. Central Intelligence Agency memorandums quoted unidentified sources as saying Kim "organized the Unification Church while he was director" of the KCIA.

That statement could not be confirmed here, and Kim did not respond to requests for an interview. Government officials familiar with his career in the early 1960s said they were unaware of any connection between Kim and the church.

Until 1968, Kim was a close aide to Park. Then there was a falling out between him and Park for reasons that are still mysterious. Sources here say Kim had not been forceful enough as Democratic Republican Party leader in pushing through a constitutional amendment to permit Park to run for a third term. He made an extended tour of Europe and resigned from Park's party.

By 1970, he was again in the party and an adviser to the president, who named him prime minister in 1971. In 1975, he again disappeared from public life. Some sources believe he had come to disagree with Park's policies. Others believe he was exiled by Park because he had become a political threat and potential challenger. Kim resurfaced again as a National Assembly member in 1978 and this year became, once again, an adviser to Park. a