Spoilers for Chapters 1 through 12ish with a hint of 15.
My guess based on context clues and Ruri and Pure's conversation is that:
In the original timeline, Pure and Akira met at a later time (college most likely). After dating for a time, they fell in love and got engaged to be married overseas. However, tragedy strikes (sounds like cancer but could be something else).
As she dies, Akira says that she wishes she could have known Pure in high school, so that they could have had more time together. Pure takes this idea and runs with it, somehow acquiring access to Ruri/Sumire's time machine, and leaping back half a dozen years into Akira's past. She then attempts to find her and rekindle their relationship. This is where Chapter 1 starts.
Worth noting that in this timeline, Ruri seems to act more decisively, perhaps spurred by his conversation with Pure. His actions and goals seem borderline insane, going so far to ask Akira "Don't you wish we had never been born?" He seems determined to ruin his parents' relationship before they fall in love, thus saving his mother from the heartbreak that leads to her death.
Some of this is guesswork. It is worth noting that Pure's memories get lost the further she alters things, so the most accurate things she says about the original timeline are right at the beginning when they meet. After that, she slowly starts thinking she was delusional and loses certainty in everything she's said (though she can remember saying it).
last edited at Mar 28, 2021 6:17PM