Diana Blackbell is a supporting character introduced as a highborn girl who crosses paths with the traveling companions Marbas and Wisteria. She is the only surviving member of the Blackbell family, a lineage of apparent significance within the world of the series. Diana has long black hair, usually tied back in a braided bun, and purple eyes, and she wears a distinctive black outfit complemented by a tiny hat.

On the surface, Diana presents a friendly demeanor. However, a deeper complexity defines her character, as she is a proud individual who has made a significant sacrifice, the nature of which is a central question surrounding her introduction. Her pride is a major driving force in her actions, yet it is precisely this trait that has led to personal loss or compromise. This inner conflict makes her a figure of sympathy and intrigue, as she appears to have much in common with the younger protagonist Wisteria, sharing a bond of youth and perhaps a sense of isolation or burden beyond their years.

Diana is a demon contractor whose partner is Naberius, a demon of considerable power who holds the rank of one of the Thirteen Calamities of the Wasteland. This connection explains the abilities she brings to the story, distinguishing her as more than just an ordinary aristocrat. Her precise supernatural skills are tied directly to this contract, marking her as a formidable presence within the narrative. Her role in the story begins when she meets the main party, and she becomes a key figure in what is referred to as the Blackbell Family arc, as well as subsequent events in London. Her personal history is also defined by her family, as she is the sister of three older brothers: Noah, Lucas, and Marius.