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