Here is the full list of all 12 Amatarasu Riddle Star achievements.
Complete the game.
Complete Melody's mini-route.
Complete Rina's mini-route.
Complete Hire's mini-route.
Complete Madori's route.
Complete Ai's route.
Complete Miu's route.
Complete Kokoro's route.
Complete Yua's route.
Choose someone to spend time with.
Finish preparations for the school festival.
Complete the sports festival.