In Japanese culture, one often refers to a peer woman or man as "sister" (ne san) or "brother' (ni san). On must be careful when referring to someone using the third person pronouns. If the other person is obviously older, one uses "onesan" for older or big sister and "onisan" for older or big brother. Here in AYA, the two are actually childhood friends and actually do consider themselves brother and sister. It's only much later that they fall in love. You might notice that she often refers to him using the "-sama" honorific which inidcates her extreme respect. He, on the other hand, as a male, almost invariably refers to her as "Aoichan" which is acceptable for when a man refers to a woman. Finally, note that both of these people are located in a relatively high social cast. She is the hieress of a major fashion conglomerate, while he is the hier to a major industrial conglomerate. (Although, he's a cast out bastard child, it's clear he'll eventually inhierit the company.) If you listen carefully, you'll notice that there's an extreme level of politeness between them and those whom they consider servents. This social behavior is typical of persons of high social status.