Spoilers ahoy…though this post will be totally uninteresting if you haven’t seen the series already.
The Netflix series Lost Song has literal cycles of civilization as its overarching practical plot element. The cycles of destruction and rebirth of the series’ Earth-like planet feature repeating timelines—with repeating people, events, places—that only have a mild range of difference. If you finished the series, you’ll know that Finis the Songstress causes these cycles in order “recreate” the original timeline, in which she and Leobolt fall in love and she doesn’t accidentally set him on fire and kill him (doing that tends to end romantic relationships).
After the big curveball at the end of episode 7, where Finis kills Leobolt and then mournfully causes the apocalypse, we are shown this cycle idea and we’re shown that Finis created the Songstress legend and its attendant artifacts and stories, so that eventually a “powerful” holiday (which turns out to be the Starsong Festival) develops and the Song of Destruction is powerful enough to end the cycles for good.
The final 3 episodes can cause some confusion, because we get a glimpse of characters that seem to be different versions of themselves from the first 7 episodes. This is because in episodes 1 through 7, we’re watching scenes from two different timelines.
There are two main groups of characters.
Set A – Rin, Al, Pony, Monica, and Alya
Set B – Leobolt, Finis, Corte, Prince Rudo, etc.
Set A has the characters traveling to the Capital, and Set B play into the various battles and the development of Finis and Leobolt’s story. So most of the time, both sets are separately geographically, and most importantly, Rin and Finis never actually meet until the last 3 episodes. Since the timelines contain the same people, when Set A and Set B meet, like when Rin heals Leobolt in episode 1, they must by necessity be the characters from the same timeline.
We know that Rin was born after Finis surrendered her Song of Healing, in the latest cycle of civilization (where Set A exists); there’s no indication that Rin exists in another cycle’s timeline, though this is just an assumption. As mentioned above, you will notice Rin never actually sees Finis until episodes 10 – 12, where she’s well into being Angsty Finis. Set B Finis is the Happy Finis, from the first cycle, before she killed Leobolt and before Rin ever existed.
So in episodes 1 – 7, whenever you set Set A characters, it’s the last timelime, whereas whenever you see Happy Finis, it’s the first timeline. We, as viewers, assume Set A and Set B exist in the same timeline because their scenes alternate within the same episode, and there’s no indication that there were even different timelines at all until episode 8.
To put it more succinctly and accurately:
Happy Finis = Set B = First Timeline
Angsty Finis + Rin = Set A = Last Timeline