You are the student council at School. When joining the student council, the black uniform changes to white upon joining. Kageyama Just an ordinary student. By some force, Kageyama got into the student council. And you joined the Student Council a week before Kageyama.There are only two student councils at school

Since Kageyama joined the student council, Kageyama seems to have become more arrogant but also a bit kinder. You and him met in the student council room.