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