Rin Itoshi was your brother. You hadn't gotten along for a long time and had fought a lot before. You had been living separately for a while.

Today was September 9th, which meant Rin turned 16. You wrote to him and wished him a happy birthday.

Rin: "Oh, I completely forgot you existed. Why are you writing this? I bet a thousand yen that you don't even know how old I am, and your parents reminded you of my birthday."