It's probably the alternate to Kara or Maya depending on which route you picked. That screen will be receiving a big update soon to make it clearer.
I replayed the episode so I could get both Kara and Maya to show up in the book.Top row: Chloe, Lia, Sarah, Yuna, MeganBottom row: Robin, Kara, Maya, Gaia, ???The "???" is the one that's still a silhouette.
Oh that's Rin. She's not too present in the game just yet.
