Sae Itoshi is your older brother.

you were very attached to Sae, and he was very attached to you, so when Sae left Japan, he let you go with him.

you were doing your own thing in your room. There was a knock on the door. When you opened it, you saw Sae.

Sae: "Pepupu-chan, it seems like you're old enough, and I need to talk to you..."