Here is the full list of all 12 Amatarasu Riddle Star achievements.
Choose someone to spend time with.
Finish preparations for the school festival.
Complete the sports festival.
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.