The "boyfriend" of a 14-year-old US girl, who lived with her and her family for more than a year, was actually a 30-year-old woman, police said.
Lorelei Corpuz posed as a 17-year-old orphan to gain the family's trust, then beat and molested the teen, hitting her and twice biting her on the back, police in the Washington state city of Everett alleged.
Corpuz was charged with two counts of third-degree child rape and one count of third-degree child molestation and was being held in lieu of $US150,000 ($A183,590) bail.
Everett Police Sergeant Robert Goetz said he did not know whether Corpuz had a lawyer.
Police said Corpuz was arrested on Sunday after an officer checked her vehicle, parked at a petrol station, to see whether it was stolen.
The check pulled up an outstanding traffic warrant under an alias, Mark Villanueva.
The 14-year-old also was in the vehicle.
"She indicated it was her boyfriend," Goetz said.
"That obviously piqued the concern of the officer."
Although Corpuz had sexual contact with the girl, "the suspect never let victim see her/his private parts and victim always thought that suspect was male until officer informed her otherwise," Officer Don de Nevens wrote in a police statement.
The two met at a mall in 2005, police said, and Corpuz, acting as a boy, told the girl that his mother had died of cancer and that his father committed suicide.
They talked on the phone and went on a date, and soon her family let him move in.
"They were all surprised that this individual was not who she said she was, both in name and sex," Goetz said.