Obtained: When Mamori reaches Stage Rank 6, you will receive a Topic from her. After you read the message, head on over to Daitama Observatory and speak with her.
During the dialog scene, select the option for Ellie to help out Mamori. When you regain control, travel to the lesson studio in Shibuya and speak to Mamori near the entrance to trigger a cutscene. Once the training begins, select the options:
- Have her take you.
- Beg her for snacks.
- Be extremely happy.
After Mamori completes the training, she can learn the “Door of Family” Radiant Skill so head on over to Bloom Palace to unlock it.