Who is Death from The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making and what is their importance?

When September travels deep into The Worsted Woods to retrieve the contents of a magical casket, she encounters Death, who manifests in the form of a small mouse-like animal and, while being sung a lullaby, falls asleep in September's arms. As her mouth drops open while she sleeps, Death reveals that she carries the casket within her, perhaps a metaphoric suggestion / evocation of the idea that all ultimate truths, all quests, inevitably end in Death.