Having never heard of this Japanese manga: NANA, I did some research online by reading the Wikipedia entry and even reading a few chapters of the English translated manga to get some insight. Nadia and Steff taking some pity on me, explained their respective characters:

It’s about two rival bands, one is a punk band and the other is a rock band. Nana [Osakai] and Layla “Reira” Serizawa (Nadia) are the respective vocalists. Shinichi “Shin” Okazaki is a bassist from Nana’s band. He kept the fact that he was underage a secret from everyone, and was actually sleeping around with women for money. When he met Reira, it started out to be like that, but they ended up falling in love. One of the things that these two have in common is that they’re half Caucasian and have had a hard time with it when they were younger. They were always hiding their relationship though, due to their age difference and their band’s reputation.

Certainly sounds interesting. Sounds like if they were Facebook buddies, their relationship status would be “It’s complicated.”