Here is the full list of all 12 Missing Stars achievements.
Find Katja in the garden and become her friend.
Join the Astronomy Club with Isolda's prodding.
Ask Natalya out on a date.
Reassure Anna that it's gonna be okay.
Enjoy the Gala with Jeanne.
Find Isolda in the woods, and scare some birds in the process.
Crash into Jeanne. Or did she crash into you?
Meet up in the catPETERia with Natalya.
Eat a sandwich and have Katja question your life choices.
Start a conversation with that one quiet girl in class.
Start playing Missing Stars.