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 Miu's route.
Complete Kokoro's route.
Complete Madori's route.
Complete Ai's route.
Complete Yua's route.
Choose someone to spend time with.
Finish preparations for the school festival.
Complete the sports festival.