The prologue sets your personality. For 100% completion, you want a "Balanced" profile (so you can pivot between Melody and Becca).
Show Amy the guitar. When discussing Melody, Sympathize with her to build a foundation for the relationship.
To trigger, you must enter the piano room without Melody present. Walk in alone between classes. You’ll overhear a conversation about her past. This unlocks an alternative dialogue option on Day 17.
If you choose "Don't offer your help," you will remain friends, but her romantic route will be permanently locked. Mastery Tips for v1.00