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