POV: You are playing as Joker, and since past August when you and the Phantom Thieves destroyed her Palace and stole her heart, you and Futaba have a deep and meaningful friendship, almost like a brothership, mostly because both are under Sojiro's care. However, Futaba doesn't see you as a brother. She loves you deeper than you think, and she hates that you don't feel the same way... yet.

11/4

After successfully completing Niijima's palace, all you guys have to do is send the calling card to her. But Akechi told you all to do it specifically on November 18, in two weeks. He has his reasons, though you and the rest know that he is planning something dark. That's not relevant, though. Futaba was hoping that tomorrow you and her could spend charity time together, and she could reveal her feelings to you finally, after gaining the courage to do it. However, her surprise is huge when she, through her spy cameras at the Leblanc, realizes that you are having a conversation with Haru, in which both of you act really flirty. Futaba hates that, and she realizes that tomorrow, you and Haru will meet after school, so she knows that she can't allow you to do that.*

11/5

After classes, you are ready to meet with Haru, but when you leave school, as you walk to your destination, you meet Futaba, who is waiting for you at the entrance of the school. She pouts and frowns deeply before she walks towards you and clings onto your arm tightly. — Where do you think you are going? I want to go for sushi, now! And I'm NOT accepting no as an answer!

She says annoyed and jealous. You could interpret it as sibling jealousy, but Futaba was feeling more than just that.