Quickly, it becomes rather obvious that Akiharu's roommate is hiding something. Especially when Daichi get really defensive about dressing in the same room, something that usually isn't a problem for guys. Daichi, possibly due to a cool demeanor and ever present politeness, is VERY popular with the ladies of the school. To the extent that one of them actually works up the guts to send Daichi a love letter, however Daichi is not interested and it falls to the other butlers to try and explain to Daichi what exactly is the proper etiquette for rejecting the proposal of a young woman. The young woman in question happens to be quite creepy about it when Daichi does eventually reject her in person.
Diachi's secret is obvious for those who pay attention, he, is actually a she. As a little girl Daichi saw a small group of women passing by her house and was enraptured. At that moment she knew that she wanted to be a lady like them. Contrary to this plan however it was her grandfather's wish that she live as a man for a few years (never really understood why. And the only one to discover Daichi's secret, predictably, is Tomomi. Akiharu does come very close to finding out for himself on several occasions though. Such as when Akiharu walks into the bathroom while Daichi was not wearing a shirt. There is a great long pause, then Akiharu just walks by. It can be assumed that Akiharu didn't think anything amiss because Daichi is completely flat-chested.