this is a list (as complete as I can make it) of what we know about kirk's backstory in tos and the tos movies, with episode sources:
- he has a brother named samuel george kirk who he calls sam. sam has a wife named aurelan, three kids (one of whom is named peter), and a terrible mustache. (what are little girls made of? and operation: annihilate!)
- he was on tarsus iv and survived the massacre. we don't know his age or what he was doing there. we only know that he saw kodos, heard his voice, wrote down what he said about the massacre, and could tentatively identify him years later. (the conscience of the king)
- when security officer mallory dies, kirk says that mallory's father helped him get into starfleet academy. (the apple)
- he was a nerd in the academy. gary mitchell called him a stack of books with legs, and he admits to bones that he was a very serious cadet. he was picked on by a fellow cadet named finnegan, who he dearly wanted to beat up but never did. (where no man has ever gone before and shore leave)
- on a similarly nerdy note, he had a big history crush on abraham lincoln and could recite the constitution from memory. maybe this is a learned skill in american schools in this future, or maybe he's just a history nerd. (the savage curtain)
- he had john gill as a history professor, who he encounters in the series as the crazy old guy who made space nazis. (patterns of force)
- kirk did not meet but looked up to captain garth of izar, a starfleet captain and explorer who was eventually decided to commit genocide and was sent to an asylum. we don't know whether this happened before, during, or after kirk's study of him or how he might have taken the news. (whom gods destroy)
- he has a deep fear of being alone and unable to act or control his situation. (many episodes, but most obviously in and the children shall lead us.)
- at some point (we have no idea how old he was or if it was before or after the academy) he nearly died from vegan choriomeningitis, a made-up disease deadly enough to be used as population control when extracted from his blood. (the mark of gideon)
- he had some kind of relationship with areel shaw, a lawyer who later ends up questioning kirk in a court martial. (court martial)
- he had some kind of relationship with ruth. when he sees her again in season 1, or a representation of her, he says it's been fifteen years. (shore leave)
- when he took the kobayashi maru, he reprogrammed the test so it would be possible to pass and passed on his third try. (the wrath of khan)
- once he became a midshipman (a cadet training to be a commissioned officer, apparently never mention as a rank outside of this series), he became close with his instructor benjamin finney. at some point, finney named his daughter jame (pronounced jamie) after him. jame is a teenager when we see her in season 1. (court martial)
- he served aboard the uss republic, where finney made a mistake that could have been disastrous and kirk logged it, ruining finney's chances for promotion. (court martial)
- he served aboard the uss farragut under captain garrovick as part of a phaser gun crew, meaning he fired the ship's phasers from engineering, not on the bridge like chekov. he hesitated before firing on a creature which killed garrovick, and blamed himself for it years later. (obsession)
- at some point, he worked with janice lester and they had a relationship. she resented kirk for being a man and having his career goals easier to achieve (we are not given strong evidence whether this is true or false), and kirk says they would have killed each other if he'd stayed. (turnabout intruder)
- at some point, he and endocrinologist dr. janet wallace got into a relationship, though it ended because they were both dedicated to their careers. (the deadly years)
- at some point, he and dr. carol marcus had a relationship and conceived david. kirk knew david existed, but carol prioritised her career as a scientist and decided to raise david away from kirk's world. it's implied that david met kirk and knew that he and carol had a relationship at one point, and he calls kirk "that overgrown boy scout (she) used to hang around with". he didn't know that kirk was his father until the movie, though. (the wrath of khan)
- shortly before serving on the enterprise, kirk taught a class at the academy, which gary mitchell was in. we don't know what he taught, only that he had a reputation for making students think critically. mitchell admits to setting kirk up with a blonde lab technician, to which kirk looks aghast and says, "I nearly married her." we don't know whether this was janet wallace, carol marcus, ruth, areel shaw, janice lester, or some other unknown woman. (where no man has gone before)
- at some point, he and gary mitchell encountered "rodent things" on dimorus which threw poisoned darts at them. mitchell took one for kirk and nearly died. (where no man has gone before)
that's it! that's all we get. everything else -- including him being born in iowa -- was from other sources added later. I just realised there wasn't one good source that was specific to the original series (star trek wikis that don't differentiate between tos, other series afterwards, and aos drive me insane), and it might be relevant to someone else too.
