Keeler ran a soup kitchen for the extreme poor of New York City in 1930. She was also a social worker who helped people find jobs when she could. Apart from from being a social worker, she was also an advocate for peace who was extremely perceptive on many topics. Her perceptive talents also extended to judging people's character. As she observed about the friendship between James T. Kirk and Spock; that Spock belonged at Kirk's side, that he'd always been there and would always be.
Leonard McCoy, who, suffering from paranoid delusions caused by an accidental drug overdose was sent into the past by the Guardian of Forever. In this condition, he saved Edith from dying in a traffic accident thus changing the course of history. In the altered timeline caused by this series of events, Edith's survival formed a pacifist movement that delayed the United States entry into World War II. This resulted in Germany perfecting the atomic bomb sooner and winning the war.
Kirk and Spock followed their friend from the 23rd century so as to prevent the change to the timeline. Upon arriving in the past, they acquired period clothes and sought shelter by breaking into the basement of the rescue mission that Edith ran. With Edith's help, Kirk and Spock were able to find work and a place to live. Spock's subsequent analysis of historical records determined that Edith had to die in order to restore the timeline to normal. Matters were further complicated when James T. Kirk fell in love with Edith. The two began a relationship. Doctor McCoy, still sick, eventually found his way to Edith's rescue mission. Edith promptly took care of him. Despite her goodness, Kirk had no option when he realized that he had to prevent McCoy from saving her life, letting her die so that the world they knew might still exist. (TOS episode & novelization: The City on the Edge of Forever)
Such an impact did Edith have on Kirk that when he found himself at the nexus of the Bajoran wormhole, the first of the Prophets to greet him did so wearing Edith's face, though Kirk initially mistook the alien for Edith herself. (ST - The Q Conflict comic: "Issue 4")
In another, Sister Edith Keeler was a worker at a free milk kitchen but preached the same philosophy of peace and kindness in her spare time as well as aiding delinquent youths reform their ways. When a truck was due to strike and kill her, she was saved by Beckwith, a time traveller from the 23rd century, her survival allowing for a German victory in World War II.
Kirk and Spock travelled back to 1930 to set things right, with both deducing Edith was the "focal point" they'd been warned about. Ostensibly to keep an eye on her, Kirk moved into her apartment block with the two eventually falling in love, much to the disapproval of Spock. After the half-Vulcan deduced Keeler had to die, she noticed Kirk becoming more withdrawn even professing her love to him in an attempt to have him open up. One night, Edith even confessed to him that she was losing her faith in mankind prompting Kirk to declare that not only was her pacifism the right path but that he loved her as well before Spock called him away. Left to her own devices, Edith gave another sermon on man's innate goodness before sighting Jim across the street, unaware of an approaching truck. Though Beckwith attempted to save her, Spock tackled him down, allowing Edith to die and for time to resume its shape. Returning to his own time, Kirk mourned his lost love, finding little solace even in the fact that she had mattered so much in the grand scheme of things. (TOS comics: "Harlan Ellison's The City on the Edge of Forever, Issue 3", "Harlan Ellison's The City on the Edge of Forever, Issue 4", "Harlan Ellison's The City on the Edge of Forever, Issue 5")
- TOS episode & novelization: The City on the Edge of Forever
- TOS - Crucible novel: Provenance of Shadows
- TOS - Crucible novel: The Fire and the Rose
- TOS - Crucible novel: The Star to Every Wandering
- TOS short story: "Triptych"
- TOS short story: "Remembering the Future"
- TOS comic: "Harlan Ellison's The City on the Edge of Forever, Issue 3"
- TOS comic: "Harlan Ellison's The City on the Edge of Forever, Issue 4"
- TOS comic: "Harlan Ellison's The City on the Edge of Forever, Issue 5"
- TOS comic: "Who's Who in Star Trek, Issue 1"
- TOS novel: The Tears of the Singers
- TOS novel: Ishmael
- TOS novel: Final Frontier
- TOS novel: Battlestations!
- TOS novel: Chain of Attack
- TOS novel: Timetrap
- TOS novel: Ghost-Walker
- TOS novel: The Rings of Tautee
- TOS novel: Assignment: Eternity
- TOS novel: Crossroad
- TOS - The Lost Years novel: Recovery
- TOS novel: Ex Machina
- TOS novel: Triangle
- TOS novel: Shadows on the Sun
- TNG novelization: Generations
- TOS novel: Preserver
- TNG novel: Engines of Destiny
- TOS novel: Star Trek: The Captain's Table (Biography: James T. Kirk)
- TOS - Star Trek: The New Voyages short story: "Mind-Sifter"
- TOS - Strange New Worlds short story: "Reflections (TOS)"
- TOS - Strange New Worlds short story: "Marking Time"