The Sum of All Fears

• Jack Ryan: Deputy Director of Central Intelligence. He develops the Vatican Treaty providing a final settlement on the Arab-Israeli conflict, but is overworked and unappreciated by President Fowler, who is unable to replace Ryan due to the latter's popularity in Congress. He also has a deep rivalry with Dr. Elliot. As a result, he begins to develop alcoholism and increasingly poor health. He later decides to retire from the agency in order to overcome his drinking problem and spend more time with his family. During the events of the novel, Ryan becomes increasingly concerned about the growing economic dominance of Japan, which becomes a theme in the following novel Debt of Honor.
• J. Robert Fowler: President of the United States. Fowler was introduced in Clear and Present Danger as the Governor of Ohio and a presidential candidate, and won against the incumbent President after he deliberately threw the election. Fowler is depicted as a former prosecutor opposed to capital punishment and in favor of broad social programs, although he angers Dr. Elliot and many of his supporters by refusing to commute the death sentences of the Ulster Liberation Army members depicted in Patriot Games. He is nicknamed "Hawk" by the U.S. Secret Service. He was widowed after his wife died from multiple sclerosis, and begins a romantic relationship with Dr. Elizabeth Elliot. He is motivated by his public image and desire to secure a legacy, and as a result takes credit for Ryan's program. After the crisis on the Denver bombing, during which his decision-making deteriorates rapidly, he was removed from office and succeeded by his Vice President, Roger Durling. According to an audio commentary in the DVD release of the film adaptation, Clancy has said that he based Fowler on 1988 Democratic presidential candidate Michael Dukakis, claiming that left-wing politicians are more likely to use nuclear weapons than right-wing ones.
• Elizabeth Elliot: Key advisor to President Fowler, and his lover. A former professor of political science at Bennington College, she was promised the position of National Security Advisor but later denied on the insistence of Fowler's running mate Durling. At the beginning of the book, she finally achieves the position using a sex scandal against Dr. Charles Alden, and nearly derails Ryan's plan by withholding a new radar system from Israel to coerce them to accept the deal. Holding a grudge on Ryan from their first encounter (depicted in previous novel Clear and Present Danger), she denies him credit for the Middle East peace plan and later falsely outs him as having a mistress, nearly breaking up his marriage. During the crisis, her advice worsens the situation and she is later placed under sedation. She is nicknamed "Harpy" by the U.S. Secret Service.
• Arnold "Arnie" van Damm: Chief of Staff to President Fowler. Van Damm is known for his informal dress style and his alertness to intrigue. Although he is personally areligious, he works with Ryan to consult the Vatican over his peace deal.
• Brent Talbot: Secretary of State. Talbot is a former Northwestern University professor of political science.
• Scott Adler: Deputy Secretary of State.
• G. Dennis Bunker: Secretary of Defense. Bunker is a decorated former U.S. Air Force captain from the Vietnam War, flying a hundred F-105 combat missions and earning three Distinguished Flying Crosses during the bombing of Hanoi. He later gained fame by constructing a large business conglomerate in southern California and gaining ownership of the San Diego Chargers, and he bonds with the President over their mutual love of football. He is killed in the Denver bombing.
• John Clark: Ryan's personal driver and bodyguard, former CIA operative.
• Domingo “Ding” Chavez: Clark's partner, CIA field operative. Works as Ryan's bodyguard while studying for a degree at George Mason University.
• Dan Murray: Deputy Assistant Director of the Federal Bureau of Investigation
• Marcus Cabot: Director of Central Intelligence. Although he is initially inexperienced and adversarial towards Ryan, he grows to respect him after the Vatican Treaty.
• Ben Goodley: Ryan's assistant and protégé. A postdoctoral fellow at Harvard Kennedy School, he became noticed by the White House and the CIA for his analysis of the Cuban Missile Crisis, and is researching a thesis on the Vietnam War. Initially manipulated by Elliot into providing classified information on Ryan, which she would later use to discredit him, Ryan's successful handling of the crisis makes Goodley rethink his opinion on his boss.
• Inspector Sean Patrick "Pat" O’Day: FBI agent who is Murray's second-in-command.
• Dr. Charles Alden: A Yale professor of history who becomes the first National Security Advisor to President Fowler. Alden is a brilliant tactician who has a good relationship with Ryan despite their differing political views. However, his tendency to have affairs allows him to be undercut by Elliot, and he dies of a stroke after his affair with a student is discovered.
• Alan Trent: "Japan-bashing" Chairman of the U.S. House Permanent Select Committee on Intelligence. Oversees Operation NIITAKA.

• Bart Mancuso: Full captain (selected for Rear Admiral) commanding a squadron of ballistic missile submarines for the United States Pacific Fleet.
• Captain James "Jim" Rosselli: Duty officer at the National Military Command Center at the Pentagon and former commanding officer of the Gold Crew of USS Maine, an Ohio-class submarine, although the real-life Maine was launched in 1994, three years ahead of the novel's timeline.
• Captain Harrison Sharpe "Harry" Ricks: Commanding officer of the Gold Crew of Maine, succeeding Rosselli. Later dies after an explosion from a torpedo launched by the Soviet nuclear-powered attack submarine Admiral Lunin in self-defense collides with his ship, causing serious damage.
• Robby Jackson: Commander of the air group assigned to USS Theodore Roosevelt, an American aircraft carrier.

• Andrey Illich Narmonov: President of the Soviet Union, first introduced in The Hunt for Red October. He attempts to carry out democratization reforms in the Soviet Union during the events of the novel but is hindered by an unstable political system. His assistance helps to bring the Eastern Orthodox Church to support the Vatican Treaty. President Narmonov dislikes Fowler, finding him to be arrogant and overly controlling.
• Sergey Golovko: First Deputy Chairman of the Committee for State Security (KGB).
• Oleg Kirilovich Kadishev: Leader of the opposition party in the Congress of People's Deputies of the Soviet Union and CIA agent (codename SPINNAKER). He unfortunately reinforces a narrative that Narmonov has no control of the Soviet military, which later convinces Fowler and Elliot that the Soviet president may have been a victim of a coup d'etat, progressively worsening their judgment of the crisis surrounding the Denver bombing until Ryan steps in.
• Oleg Yurevich Lyalin: A KGB illegal based in Japan who has a well-developed network of agents codenamed THISTLE. He uncovers the Japanese Prime Minister's corrupt dealings with Mexico over trade agreements and offers his services to the CIA, which accepts him (codename MUSHASHI). Based on his intelligence, Ryan launches Operation NIITAKA, which later becomes instrumental in blackmailing the President of Mexico into allowing the arrest of Qati and Ghosn in Mexico City after the Denver bombing. Unfortunately, he was caught by the Soviets and charged with treason, but Ryan pleads with Golovko to free Lyalin since he did not reveal Soviet state secrets.
• Valentin Borissovich Dubinin: Captain First Rank of the Soviet Navy and commanding officer of the Akula-class submarine Admiral Lunin.

• Ismael Qati: Palestinian terrorist and mastermind of the bomb plot. An experienced field commander for the Popular Front for the Liberation of Palestine (PFLP), he is suffering from terminal cancer, which drives him to carry out his mission. After being captured by Clark in Mexico City, he was later executed in Saudi Arabia.
• Ibrahim Ghosn: Palestinian terrorist and one of PFLP's experts on explosives and electronics. It is stated that he nearly received a degree in engineering from the American University of Beirut but was prevented from formally completing his studies due to the 1982 Lebanon War. Allied with Qati, he is also executed along with him in Saudi Arabia.
• Günther Bock: German terrorist and ally of Qati and Ghosn. A former Red Army Faction member, the collapse of Marxism, coupled with the arrest of his wife and fellow terrorist Petra Hassler-Bock as well as the loss of their children, forces him to go into hiding. After learning about the Soviet Union's cooperation with the United States in combating terrorism and Petra's suicide, he is motivated to trigger a nuclear war with the U.S. and Russia by arranging a false-flag attack on American and Soviet forces in Berlin. Later killed during the battle and was identified by the German police.
• Marvin Russell: Professional criminal, drug dealer, and activist for the American Indian Movement as a Sioux Indian living in the Badlands. Along with his brother John Russell, he had a history of juvenile delinquency while living with their impoverished family at a reservation in Minnesota and spent a year in prison. He developed a resentment towards the United States. After his brother is killed in a botched FBI operation, he travels to Europe to connect with international terrorists, gaining the trust of the PFLP by killing an undercover Hellenic Police officer in Athens and by comparing the Indian Wars to the Israeli–Palestinian conflict. He helps Qati and Ghosn enter the United States with the bomb. Deemed as a security risk, he was murdered by the two Palestinian terrorists in a Denver motel on the day of the nuclear detonation, but their sloppy work allows the FBI to ascertain the circumstances behind the bomb.
• Dr. Manfred Fromm: German engineer and terrorist who is recruited by Bock to assist in rebuilding and improving the captured Israeli nuclear weapon. Later executed by Bock's men after his services were complete, unaware that Dr. Fromm was not yet finished with the bomb.

• General Abraham "Avi" Ben Jakob: Assistant director of the Mossad, Israel's foreign intelligence agency. Depicted as a friend of Ryan, he helps convince the Israeli Cabinet to accept the Fowler Plan after meeting with him despite his distrust of the U.S. government.
• Rafi Mandel: The Defense Minister of Israel. Mandel initially opposes the treaty but accepts it after admitting that Israel's security status has improved from it and after Prime Minister Askenazi promises to support his planned bid for Prime Minister
• David Askenazi: The Prime Minister of Israel.
• Captain Benjamin "Benny" Zadin: An Israeli National Police officer in command in a demonstration at the Temple Mount in Jerusalem. Embittered by his personal problems, he kills an unarmed demonstrator there. Coincidentally the younger brother of Lieutenant Mordecai "Mutti" Zadin, an Israeli Air Force pilot who was shot down by Syrian surface-to-air missiles during the Yom Kippur War, unaware that he was carrying a nuclear weapon. The younger Zadin is ultimately committed to a mental institution.

• Prince Ali bin-Sheik: Crown Prince of Saudi Arabia
• Ayatollah Mahmoud Haji Daryaei: The Supreme Leader of Iran. Opposes the Vatican Treaty, and is falsely implicated by the terrorists in the Denver bombing.
• Hashimi Moussa: A 20-year-old Arab sociology student in Jerusalem and the leader of the demonstrations. Hashimi, who bears physical scars from police beatings, convinces the demonstrators not to enter the Temple Mount and adopt nonviolent protest, resulting in Israel losing international sympathy. Hashimi is killed by Captain Zadin.

• Bob Holtzman: Senior White House correspondent for The Washington Post who is being used by Elliot to discreetly out Ryan as being suspected of financial and marital improprieties. After being confronted by Clark, he works to expose Elliot's part in the whole affair.
• Cathy Ryan: Ophthalmic surgeon at the Wilmer Eye Institute, which is part of the Johns Hopkins University School of Medicine; Jack Ryan's wife who desperately wants a third child but was blindsided by her husband's alcoholism and work. Later becomes troubled with reports of her husband having a mistress, but is talked down by Clark and Chavez. She then proceeds to publicly embarrass Elliot, who is her former professor at Bennington College, at a White House dinner. She becomes pregnant with Katie by the end of the novel.
• Carol Zimmer: Hmong American widow of Sergeant Buck Zimmer (detailed in Clear and Present Danger). Ryan helps her and her family through an educational trust fund as well as setting up a 7-Eleven convenience store to supplement her family income. This was misinterpreted by Elliot as indication that Zimmer is Ryan's mistress.
• Solomon Mendelev: An Orthodox Jewish rabbi from New York City who accuses the U.S. government of trying to subjugate Israel through the Treaty. Although he has never set foot in Israel himself due to his religious beliefs, he sparks widespread opposition to the Treaty among the Israeli public, and Dr. Eliot's attitude towards him worsens Ryan's relations with her.

The title is a reference to nuclear war and to the plot by the novel's antagonists to reconstruct a lost nuclear weapon. It comes from a Winston Churchill quote serving as the first of the novel's two epigraphs:

Why, you may take the most gallant sailor, the most intrepid airman or the most audacious soldier, put them at a table together—what do you get? The sum of their fears.[3]

The Vatican-like solution for Jerusalem, which was implemented in the book, is ultimately derived from the 1947 United Nations Partition Plan for Palestine, which indeed provided for making Jerusalem such a "Corpus separatum" (Latin for "separated body"). The course which the 1948 Palestine war took prevented implementation of this plan. In later years, various peace plans and diplomatic initiatives sought to revive the idea, but in reality it has never come close to implementation. The plan is known for being popular outside the Middle East, but unpopular among the actual residents of Jerusalem, who would prefer that their "side" should rule entirely rather than submit to a neutral administration.

A database file with certain limited details about John Clark is included as background information within the first Rainbow Six game, and moreover, the same database entry is also found in many of the sequels. That entry mentions in passing that "the Denver, Colorado atomic detonation [occurred] in 1989". That information might not be canonical, since the book is set after both the fall of the Berlin Wall (November 9, 1989) and possibly the First Persian Gulf War (January–February 1991). If it is canonical, though, this means that the book is not set in the same year it was published. A second inference is that 1989 was likely the year in which President Fowler's administration ended.