Death Three
Sayu tries to be normal, to smile through her anger for her new family, she honestly tries. But reliving the same formative years a third time, even if it is in a Japanese household, is irritating.
Sayu loves her new family: her doting mother, her hardworking father and her genius brother. Only, she can't help but flinch when her brother comes near her sometimes. She remembers watching him in both of her previous lives succumb to the seductive power of a Death Note. She remembers how he became Kira and she's afraid because she is not their Sayu.
So when she is out with friends and Lind L. Taylor dies onscreen she knows what she has to do, canon be damned. It takes a few months, but she is patient, and steals a Death Note page. She follows her father to one of the many hotels for the investigation and hides her anger at Light and fear of him with a smile as she confronts her upset father, his team and an unperturbed L. She continues to smile even as she crushes her father's image of Light. Her smile doesn't falter as she pulles out a pencil and the page of the Death Note to write something on it before handing it to L. It stays strong even as he reads the page and his eyes flick to her as she hugs her shocked father. Her smile only falters when she grips her chest in pain and watches her father cry as he holds her.
She was fourteen when she wrote her name on a page of a Death Note and died of a heart attack, hoping to be released from this neverending hell.