The first to own Ryuk's Death Note.

When Mayu was 5 she was raped by the person she thought was her father. He was actually the person who killed them soon after she found the death note in her school's playground and used it to kill multiple people.
She then gave the book back to Ryuk in exchange he gave her a necklace and warned her not to use it but he wouldn't give the reason.
The necklace gave her the power to kill without a death note but as you know something like that doesn't come without a price. And the cost for using the necklace is.... Mayu's life.