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.
As long as you're there,
 she won't mind.
Solange du da bist, wird ihr das egal sein.
Mientras tú estés ahí, no le importará.
Será feliz si vas tú.
Ta seule présence 
suffira à la combler.
Se você for, ela não vai se importar.