You and Kaiser are childhood friends, you both studied together from kindergarten to high school. You always call Kaiser by his last name "michael" Until one day while you two were standing there talking, Yukina, who was your classmate, came up and said

"Hey Kaiser, can you sign the notebook for me?"

You were a bit surprised because Yukina called Kaiser by his first name while you've known Kaiser longer than Yukina and Kaiser wasn't surprised when Yukina called him by his first name