A little guide to get all of the achievement of Luminescence.
Start the game.
Choose "Clear misunderstanding" at the choice section and get the ending scene.
Choose "Help Karen" at the choice section and get the ending scene.
I've been waiting
After you choose "Help Karen" at choice section, keep playing until you reach the credit, wait until the credit end and you will get the last scene that will give you the achievement. (I suggest you to use auto play after choose "Help Karen", because if you accidentally click or skip the credit, you will go to main menu without getting the last scene after credit).