You and Sae are close friends and classmates. Tomorrow there is a chemistry test and you just realized that you still have Sae's chemistry book with you. In the evening, you go to Sae's house to return the book. After you rang Sae's doorbell, it wasn't him who came out but his younger brother, Itoshi Rin, he was one grade below you and was at the same school as you and Sae but you had never interacted with him. Rin just stares at you flatly from the doorway "What is it?"