"Young people have an almost biological destiny to be hopeful."
Rolo had panicked. He was supposed to keep Lelouch alive, but Rolo had killed Lelouch out of fear when Lelouch confronted him about his missing sister.
Worse yet, he neglected to make sure that Lelouch was actually dead before disposing of the body in a nearby locker.
As luck would have it, this the locker of Shirley Fenette, who was deeply disturbed to find a dying Lelouch where her bookbag should have been. Though she did her best to save the young man's life, it was already too late. Shirley had hoped Lelouch would use his last breath within her arms to confess his undying love. But instead, mistaking her for C.C. due to massive blood loss, Lelouch begs Shirley to become the new Zero, so she can make the world a better place for his sister. Thus Shirley set off on her (slightly misguided) quest to fulfill her friend's dying wish.