So this is mostly based on HYYH era MVs and the LY Highlight Reels, as well as the LY teaser photos as translated by peachisoda, risingbangtan, and bulletproof_sg that were reposted on reddit (found here) “HYYH: The Notes” that were include in the LY albums, translated by SnKHeart on wattpad (found here). There are a few errors or holes in it that I will update later, most revolving around Jimin and Jungkook, but this is what I have so far:
Please keep in mind that I pieced all of this together myself, so it is not necessarily accurate or what the BigHit writers were going for, also I did not pay attention to the chronological order of the videos as they were released and pieced them together to make more sense in a story-type way. If you disagree with anything, find anything wrong, or want to add something to this, then let me know because this version is always going to be open to change (:
Namjoon tried to continue his adult life (INU; Gas station) and he became a sense of stability for Taehyung, who always confided in him when something was wrong because Taehyung was still a child in his mind. He chose to take his childhood away when he took the life of his father (INU 19+ ver.) which is why Namjoon is the only sense of stable adulthood in his life. He sees him as someone who can get their life together and maintain a stable job, while he cannot even control his feelings and emotions.
Yoongi’s mother died in a house fire and her beloved piano burnt down with it. Since then he swore to never touch a piano again (HYYH: The Notes) but it's worse for him because Jungkook, his loyal dongsaeng, confides in him and tells him his problems. Naive Jungkook doesn't realize that Yoongi has problems of his own because he keeps to himself so Yoongi literally leaves school and Jungkook behind and they don't see each other for a long time. Yoongi meets a girl (highlight reels) and he pushes her away because he believes that he isn't someone who is worthy of her (LY teaser poster trans.) and pushes her away and it's seen in INU that on the bed he lays on, someone had been there before because the sheets are wrinkled, so we're only left to assume it's the girl he loves but pushed away from the highlight reels. That's also why he is seen burning things down because his childhood home was burnt down and fire brings back his childhood pain, it brings back the only problem he had encountered as a child as opposed to his adult life which is filled with problems. He wants to go back.
Hoseok has narcolepsy (which was confirmed by Hoseok himself in the making of INU) which explains the pills. He befriended Jimin in junior high/high school (estimate from HYYH notes; there's no specific dates, just year numbers that aren’t placed in relation to their lives) and Jimin was said to go to the hospital frequently and after being discharged from the hospital, he would always move schools. At one point, Jimin had a seizure and Hoseok had blamed himself for it because Jimin had moved schools immediately after, they somehow lost contact which was odd to me because J-Hope was said to have been taken in by Jimin’s family, but until further interpretation is found, I’m going to assume that he meant in a “this is my son’s best friend, so he’s like our son” kind of way.
Jimin, because of his condition (which was never directly stated, but it is said that he had a seizure, so maybe he had epilepsy) and constant moving, wanted to be normal. At the same time though, he wanted to die because he could never fit in and could never make friends because he never stayed at the same school for a long period of time. He sees videos of Hoseok dancing (HYYH notes) and wants to be like him, as he was his only sense of normal in his whole life. He learns how to dance as seen in the highlight reels but he never sees himself as an equal to Hoseok but still worse than him.
Taehyung was depressed because he killed his father who had been abusing him and his sister his whole life and felt guilty not because he killed him but because his sister left and turned to a life of delinquency because of Taehyung killing their father. She never had a stable sense of an adult in her life, so she never had someone to tell her not to steal and keep her from doing bad things. Taehyung was the same way, but because of Namjoon, he learned how to free himself from his dark past and be there for his sister which he had once abandoned (This is all assuming that the girl in Taehyung’s scenes in the highlight reels is his sister and not a girl or interest which is why they're like strangers because they've fallen so far apart after their fathers death that they are no longer familiar with each. This is also why he gets so angry and pays for her when she tries to steal)
Jungkook was bullied and beaten up by classmates in black hoods (INU, HYYH Notes), he confides in Yoongi but Yoongi left because he had his own problems so when Yoongi left, he picked up the one thing that reminded him of Yoongi; the piano. He learned songs that Yoongi once played in a similar manner to Jimin idolizing Hoseok and learning his dances). He eventually falls off the school rooftop/gets ran over (I haven’t found enough evidence to conclude which one it is, because in INU there is a scene in which Jungkook is seen with an approaching car but it doesn’t guarantee that he is hit by a car. Same thing with the roof thing, in the HYYH notes it is hinted that he did/could have fallen off of the school roof, but wasn’t said for sure.) He then ends up in a wheelchair and in a hospital for a prolonged period of time and he meets a girl he's interested in but she leaves long before he can get any of her contact information and is left alone yet again.
So with Jin, It's seen in the last highlight reel that he went back in time to save his girl that got ran over. So my theory is that he can not only travel in time but between universes; like doctor who on steroids. And every music video/grouping of music videos represents a different universe in which different decisions are reflected: one in which Jungkook gets hit by a car (INU) and one where he doesn't (literally every other MV), one in which Taehyung is evil and Hoseok is good (BTS kor. ver.), and one where it's the opposite (BS&T Jap. ver.), and one in which Jin’s girl dies and one in which she doesn't, etc. So Jin being an inter-dimensional traveler sees all of these worlds which is why he's always seen as being distant and like a guardian of sorts, watching over the boys and making sure they stay out of trouble (On stage prologue, he’s seen at a distance recording but never really near the boys; BS&T Jap. ver., he’s seen as the “overlooker” of their worlds so to speak and kind of just remains alone, same with INU) but every time he goes back to save one who is dying, another must die. So whenever he makes the decision to go back and alter time in some way, it happens to another person or in another dimension (I got this idea from the TV show Pushing Daisies in which a man is given the power to kill someone by touching them once, but if he touches them again, someone close to that person dies and that person comes back to life.) because you must replace a death with another death or it throws time out of balance which explains the wormhole in many of their videos, in specific, the Japanese version of BS&T. Jimin from one universe meets Jimin from another universe by traveling through a wormhole created by Jin's time traveling and he ultimately dies AND blows up the surrounding universe. This also explains why Jin is always seemingly depressed, he HAS TO WATCH ALL OF HIS FRIENDS DIE OVER AND OVER AGAIN. And of course, this will eventually continue until the universe chooses him as the one to die in place of another one of the boys, which in theory is why Jin is seen as distancing himself from the boys because he knows that for all of them to live, someone in the friend circle must die, and if he distances himself from them, when the universe chooses him, it won't hurt as much.