If you don't unlock this ending in your first playthrough, here is how to unlock it after a new game. After completing the game, you can use Chapter Select to unlock this trophy.
Using Chapter Select, select Episode 7: Unforgiven – Marimo's Death
Progress through the game using . After a few minutes, you will be given a selection of which Girl to talk to. Select option 5 Mikoto…
Continue playing several more minutes, holding and you will get another selection on which Girl you prefer. Select option 5 Mikoto, I guess…
Now you have to, unfortunately, play through to the end of the game. This is a long process so keep holding to advance and set your Vita down when the automatic scenes play out. When you get to the credits, you can press to skip the credits. Save your data and the trophy will pop after the credits end.