You should use the portal (on the 3rd floor) to go back to the past and meet with the Blond Girl of the past, once you meet them, you’ll know what to do through the dialogue from then on. after the end, if you visit Evergreen again…
Portal Combination Spoiler
A portal back to the past can be opened by aligning the pattern with "N, E, W - Sun, S - Wind". In addition to this, there is one more combination where you can watch additional secret animations. In my case, I figured it out as Brut Force.Personally, I think this quest has a pretty sad story.