TENSHI JA NAI!! (I'm No Angel), by Takako Shigematsu. First published in 2003 and first published in North America in 2005.
PLOT:
Hikaru just wants to be left alone. She modeled briefly as a child and all she got out of it was a lot of attention from bullies. Ever since, Hikaru has done her best to coast through her school years without attracting any sort of attention. Her plans go to hell when her new all-girls' school decides to have her room with Izumi Kido, a popular actress and idol singer. Hikaru soon learns Izumi's secret: he's a man who only poses as a woman. Both Izumi and his manager Yasukuni are determined to keep this secret at any cost, and they are not above blackmailing Hikaru to achieve this. It seems that between this, jealous classmates, and some family secrets of her own, Hikaru is going to need to make some all-new plans if she's going to survive.