Imouto wa Shitteiru

At work, Kiichiro Miki is an ordinary salaryman who is considered expressionless and dull by his co-workers. He floats around at drinking parties and doesn't seem to be having any fun at all. But only I, his younger sister, knows the truth about him. My brother is actually “the most interesting man in the world."

(Source: Kodansha, translated)