After reading the whole novel, I kind of understood the whole plot with these endless 20,000 year cycles, but I still have one question.
Who the ♥♥♥♥ is Setsuna?
In terms of where his story began, was he born in a winter world, or a summer one?
Who was he before he lost his memory?
From summer routes it's pretty clear that he has knowledge that can only be obtained in summer time period, like the history of Japan, the names of old games and so on, so I guess he was born around 1980? But I can't imagine how the cycle can start if Setsuna was born at this time, and if he was born in the winter period, how does he know all this?
No matter how hard I think, I can't find the answer, because it seems to me that this whole cycle has no clear beginning, which is the biggest plothole of the story.