Excerpt: As the disturbing portrait of an obsessed and driven killer emerged from a prosecutor's opening statement in Ontario Superior Court yesterday, a teenaged girl, owlish glasses on her chubby cheeks and her long hair in a ponytail, sat serenely at a table near the front of the room.

Jarringly, the alleged mastermind of the first Toronto homicide of 2008 and the girl are one and the same.

She is the 17-year-old who can be identified, under the provisions of the Youth Criminal Justice Act, only as M.T. She is pleading not guilty to first-degree murder in the slaying of Stefanie Rengel, a younger girl it appears she barely knew but hated anyway.

M.T. is not alleged to have been present when Stefanie, who was just 14, was savagely stabbed six times outside her east Toronto home on New Year's Day last year.

That dubious distinction belongs to her then-boyfriend, D.B., now 19, but at the time just days from his 18th birthday. He faces a trial of his own, on...