Ms Maxine Carr, the ex-girlfriend of Mr Ian Huntley, faces cross-examination today by the Soham murder trial prosecution.
Ms Carr has already admitted to the court that she lied to police and journalists for two weeks to protect Mr Huntley.
When Ms Carr appeared in person for the first time at the trial yesterday she described how she and Mr Huntley, the caretaker accused of murdering schoolgirls Holly Wells and Jessica Chapman, concocted a false story about the night the 10-year-olds died.
Ms Carr (26) was a teaching assistant at Holly and Jessica's school and is charged with assisting an offender and conspiring to pervert the course of justice in the case.
Mr Huntley (29) denies murdering either girl but admits Holly died in the bathtub of the house he shared with Ms Carr. He told the Old Bailey Court on Tuesday that Jessica died when he put a hand over her mouth to stop her from screaming.
Ms Carr was visiting her mother in Grimsby the night the schoolgirls died in August, 2002, but when asked in court why she told police and journalists she was at home with Mr Huntley that night she said:
"Because I did not want Ian accused of anything he had not done. Even though I knew it wasn't right in moral terms, I thought I was doing the right thing at the time for that person," she said.
She said she did not know Mr Huntley's version of events - that the girls died in their house - until it was put to the jury.