Here is the full list of all 12 Amatarasu Riddle Star achievements.
Complete the game.
Complete Rina's mini-route.
Complete Madori's route.
Complete Melody's mini-route.
Complete Hire's mini-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.