Gary Seven was the code name used by Supervisor 194 of the mysterious alien group known as the Aegis. He was a human male, born as Caleb Howell on a cloaked planet in System Zeta Gamma 537 in 1932, the descendant of humans captured by the Aegis circa 4000 BC, in order to train them as agents to aid in the development and survival of humanity. (TOS episode: "Assignment: Earth"; TOS novel: The Eugenics Wars; TOS - Year Five - Weaker Than Man comic: "Issue 17")
Howell was called in by the Aegis five days after his 35th birthday by a man. He given his the assignment of protecting Earth and to safeguard its history. He was given the name Gary Seven and partnered up with Isis. (TOS - Year Five - Weaker Than Man comic: "Issue 17")
Seven took on many assignments for Aegis. In 1206, he was an advisor and friend to Genghis Khan about conquering the world. In 1588, he helped the British defeat the Spanish Armada. In 1914, Seven stood by as Gavrilo Princip carried out his assassination of Archduke Ferdinand. (TOS - Year Five - Weaker Than Man comic: "Issue 17")
Seven was sent back to Earth in 1968 to complete the work of Agent 201 and Agent 347, who were killed in an automobile accident. Before reaching Earth, his transporter beam was intercepted by the USS Enterprise, who were orbiting Earth on a research mission. Seven managed to escape to the agents's New York City apartment.
With assistance from his aides Roberta Lincoln and Isis, and in spite of hindrance from Captain James T. Kirk, Seven managed to stop mankind from launching into a nuclear war. (TOS episode: "Assignment: Earth")
Upon returning to headquarters however, Seven furiously confronted Isis about her earlier falsehood, having seen the incontrovertible evidence that 201 and 347 had already completed their mission prior to their demise. Isis noted that the mission had been necessary to introduce Seven to Kirk before informing of Aegis' true agenda: to freeze the societal development of the Milky Way galaxy by making the consonant Tholian Assembly the sole power. (TOS - Year Five - Weaker Than Man comic: "Issue 17")
The two agents were responsible for preventing Professor Tepesch from brainwashing John F. Kennedy and Jacqueline Kennedy, as well as apprehending the Watergate burglars in Washington, DC. (TOS novel: Assignment: Eternity)
- This is in reference to British television series The Avengers, specifically the episode "The Cybernauts".
In the 1970s, the incident at the Three Mile Island nuclear facility was lessened by their intervention, as was an incident that involved the crashing to Earth of Skylab. (TOS novel: Assignment: Eternity)
In 1989, Seven and Lincoln prevented an alien incident from causing a planetwide panic as a unknown extraterrestrial landed in the USSR, as reported by TASS. TASS also reported that the USSR lost 20-30 Hind-24 gunship/assault helicopters fighting the lone UFO. 
On stardate 6021.4 in 2269, Seven created a wormhole that brought the Enterprise from Federation space into Romulan space. Seven ended up stuck on a planet used as the base of operations for Agent 146 (a.k.a. Septos) with James T. Kirk, Hikaru Sulu and Pavel Chekov and without other assistance. Later, assistance was found in the form of Seven's associate, the tiger-like, rhinoceros-like Osiris. (TOS novel: Assignment: Eternity)
Romulan agent Commander Dellas killed Septos, and then Osiris, before Seven could stop her. When Dellas later attempted to kill Spock at the Khitomer Conference in 2293, Seven sent her into the past with his servo. Seven told Kirk he knew Dellas as she was the only Romulan who covered her face at the Conference. (TOS novel: Assignment: Eternity)
- Sarek revealed that all the Freelans at the Khitomer Conference covered their faces.
Seven once assisted the scientists later dramatized in the 1973 film The Andromeda Strain.
The next day, Seven hired a "today only" secretary to cover Lincoln's duties. This woman was called Allison and was not privy to the true work done by the agents.
By 18 May, Seven had infiltrated Sarina Kaur's Chrysalis Project, but gotten caught and locked in an animal cage. When Kaur activated Seven's servo, Lincoln answered the hail, blowing both their covers. Later that day, Seven escaped the cage and destroyed Chrysalis. Kaur refused to leave and so died with her project.
On 1 November, 1984, Seven took Khan away from the riots caused by Indira Gandhi's assassination, after which Khan told Seven he owed Seven a debt. Seven returned Khan back to India two days later, but recruited Khan for a mission on 2 December of that year to the Da Vinci Research Base in Antarctica to visit Wilson Evergreen. Evergreen resented the visit and attacked Khan, who stabbed him to death in response. After Evergreen got up unharmed (being Akharin, aka Flint), he was told by Seven that his research had been modified by his sponsors to create, and not close, holes in the ozone layer.
On 10 October, 1986, Seven and Khan infiltrated Vladimir Lenin's tomb in Moscow, but Seven was captured by the guards. Khan then rescued him, repaying his debt. That evening, Lincoln mentioned Seven's friend, Robert McCall.
After Khan began his visions of world domination on 4 December, 1984, Seven encrypted the collected and encrypted data on Chrysalis held by the Beta 5 computer, though this later proved futile. In the 1990s, through Roberta Lincoln, Seven had advanced technical data funneled to Project F at Area 51 in order for them to complete the creation of the DY-100 transport. This was partly because Seven intended to use the craft as a means of sending the last remnants of humanity to a new world in case he failed to stop World War III. Furthermore, he was heavily involved in the shadowy conflict of the Eugenics Wars that was being waged by the various genetically engineered supermen from the Chrysalis Project which included Khan Noonien Singh. He used information and intelligence sent to him by Isis, who was disguised as Khan's advisor Ament, to force the various supermen to fight against one another, thus destroying each one of them in turn except for Khan himself. With Khan the last survivor and the world against him, Singh intended to use his Morning Star satellite weapon to destroy the Earth's ozone layer and exterminate the entire human race as a result. However, Seven had Roberta Lincoln steal the DY-100 prototype and used it to exile Khan and his followers from the planet in exchange for the deactivation of Morning Star. Though Khan agreed and had his followers board the SS Botany Bay, Joaquin Weiss, his bodyguard, resisted and threw a dagger at Roberta Lincoln. However, Isis turned to her human form and jumped in the way of the blade, thus sacrificing her life for Lincoln. This left Gary Seven greatly saddened and bitter, as he believed that he should have left all the genetically engineered children of Chrysalis to die with the project instead of saving them. Though Khan offered the life of Joaquin, Seven refused to take another being's life and instead allowed Khan to depart the planet. This saw the end of the Eugenics Wars in the year 1996. (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 2)
Afterwards, Roberta Lincoln asked if they had unleashed Khan on the Universe, to which Seven replied that the cosmos had survived things far worse than Khan Noonien Singh. He later informed his protégé that he was leaving and intended for her to be his replacement to guard the planet Earth. At the time, a mist of purple light unveiled a tomcat that Gary Seven revealed was Roberta's partner, Ramses. He later departed the planet in order to get time to mourn the death of Isis, whom he believed deserved better than the grim death she had received. (TOS - The Eugenics Wars novel: The Rise and Fall of Khan Noonien Singh, Volume 2)
On 11 September, 2001, Seven prevented Shaun Christopher from boarding one of the planes which crashed into the World Trade Center in New York City. As a result, Christopher survived to command the first manned mission to Saturn aboard USS Lewis & Clark in 2020, a pivotal event in the history of human space travel. (TOS short story: "Assignment: One")
Seven later used his time-travel abilities to visit the 23rd century. He journeyed to the starship Enterprise over the planet Sycorax on stardate 7004.2 where he gave his own opinion about the UFP's debate over the entry of the genetically engineered inhabitants of the Paragon Colony. Seven explained to James T. Kirk the dangers of genetically enhanced beings and his belief that it was not DNA that decided a person's values but their teachings and morals. Afterwards, he once more departed into a purple mist for parts unknown. His words had a great impact on Captain Kirk, who decided that the Paragon Colony would not be admitted into the Federation but instead face a quarantine, and that they would gain protection but would otherwise be isolated from the rest of the Federation. (TOS - The Eugenics Wars novel: The Rise and Fall of Khan Noonien Singh, Volume 2)
Back in the 21st century, Seven and Isis were secretely aboard the T'Plana-Hath during Earth's first contact with the Vulcans. Seven and Mestral witnessed the launch of Friendship One from the control center in New York City in the year 2067. (TOS - Year Five - Weaker Than Man comic: "Issue 17"; TNG novel: Hearts and Minds)
In 2270, Seven and Isis had begun manipulating the Tholians, providing them with the technology necessary to overcome the United Federation of Planets. (TOS - Year Five - The Wine-Dark Deep comics: "Issue 7", "Issue 8")
After the Enterprise had left I'Qos, Seven beamed aboard, sabotaging the ship to force an evacuation as he confronted Kirk. Though Kirk proved the better fighter, Seven pulled out a hidden blaster to force his surrender. Before Seven could force Kirk to crash the vessel, Spock managed to beam aboard, the two managing to force Seven to retreat. (TOS - Year Five - The Wine-Dark Deep comics: "Issue 11", "Issue 12")
Continuing to sow chaos across the Federation, Seven manipulated Harry Mudd, the Originalist front runner for presidency, to drop out of the race and suggested that Renei, far more passionate about the cause's xenophobia, run in his stead. (TOS - Year Five - Weaker Than Man comics: "Issue 14", "Issue 15")
Isis was later sent to unleash a plague on Proxima Centauri only to be killed by Hikaru Sulu and Pavel Chekov, her absence having a detrimental affect on Seven's sanity. After the Enterprise had stumbled onto a Tholian stasis tower on Vulcan, Spock was sent back in time and changed Vulcan history. With the trap not ready to be sprung, Seven beamed aboard the Enterprise and advised the crew what to do, though swearing that he would his revenge. (TOS - Year Five - Experienced in Loss comic: "Issue 21")
Following the completion of the Enterprise's five-year mission, the Tholians begun their invasion. Seeing potential in Spock, Seven abducted him and showed him the chaos that the Federation would cause over the next nine centuries. Unconvinced by Seven's dogma, Spock beamed away and reported his coordinates to Kirk. When the captain arrived, Seven realized that Isis had kept him in line with Aegis' rules and he could end Kirk's legacy by killing him in the past. (TOS - Year Five - Experienced in Loss comics: "Issue 22", "Issue 23")
Using his servo, Seven travelled back to Kirk first taking command of the Enterprise only to be met by the Kirk of 2270. Stunned, Seven travelled ever further back to key moments in Kirk's life only for Kirk to always beat him there. At wit's end, Seven travelled to the USS Kelvin in 2233 where Kirk was again waiting for him. After the two briefly came to blows, they found common ground in the sense of purpose they derived from their respective missions. When Seven agreed to stand down, Kirk brought him to the moment of their confrontation in 2270. At Kirk's advice, Seven approached the younger Kirk and gave the captain his servo, telling him exactly what to do. In the aftermath, Seven surrendered himself to Starfleet Medical in the hopes that they could undo his body's forced homeostasis and allow him to live a normal, human life. Though he was held in stasis at the time of the Presidential election, Leonard McCoy was confident that Seven would be living a normal life on Earth within a year's time. (TOS - Year Five - Experienced in Loss comic: "Issue 24")
Physical health and description
Having a textbook intelligence quotient of almost 200, Seven was at the peak of human, non-augmented performance. He was able to tolerate and resist a Vulcan nerve pinch. (TOS episode: "Assignment: Earth")
Even in later years, his vitals matched those taken more than twenty years earlier, by Doctor McCoy. Fluent in many domestic and extra-terrestrial dialects, Gary was able to comprehend and reply (in English mostly) to Isis, and later Osiris.
The writers of his episodic appearance showed Gary Seven's knowledge of extraterrestrial events, as evident when he recognizes Spock as a Vulcan, stating aloud that Humans and Vulcans have yet to meet, in 1968. Later sources established that Gary's Aegis masters extend their operations throughout time, meaning that their time travelers would have foreknowledge of the eventual first contact at Bozeman, Montana in the 21st century.