Цудзукі Май

An ardent fan of Blast since its inception, Misato loves Nana and has befriended the band. However, "Uehara Misato" is not her true name. Misato/Mai claims that a fortune teller told her that if she used the name "Uehara Misato" she could become closer to Nana, but may have actually chosen it because it's the name of Nana's half-sister. Mai later joins the staff at Gaia, Blast's record label. She came to know about Nana and Blast when she read her grandfather's diary and discovered that he and Nana's grandmother were once lovers.

Recent chapters reveal that Mai may actually have a secret she prefers not to reveal since the tabloids have discovered Nana's father's surname may actually be "Tsuzuki", which would imply that she, like Uehara Misato, is Nana's half-sister. (Source: Wikipedia)