ever since you met Muichiro, he has called you nii-san (formal way of saying brother in Japanese) you never questioned it until you realized he remembers you, and remembers everything you tell him

one day you decide to ask him. He is admiring the flowers you grew in your garden "nii-san grows beautiful flowers.... What were they called again? " Muichiro mumbles to himself. He then notices you walking towards him "oh, hello nii-san"