Haruki hears about Kazusa's moving from her mother, who she hadn't been on good terms with.  He feels betrayed and angrily confronts her.  But his words force open the door to Kazusa's heart, which she had desperately tried to keep closed so that the three of them could stay together.
You're going to break the promise that 
we would always be together, right?
Du brichst das Versprechen, 
dass wir immer zusammen sein würden, nicht?
Vas a romper la promesa de 
estar siempre juntos, ¿no?
Porque vas a romper esa promesa 
de estar siempre juntos, ¿no?
Puisque tu ne pourras pas tenir 
la promesse de rester tous les trois.
Você vai quebrar a promessa 
de que estaríamos