A descendant from a long line of aristocrat witches, Diana demonstrates high aptitude for magic at a young age. From a young age, she had a great admiration for Shiny Chariot, but because of Chariot's bad reputation in the magical world, she kept her admiration of Chariot secret and didn't attend any of her shows. However, her mother arranged things so she could go secretly to one of her shows in Japan, the same show as Akko attend. Diana was enchanted by the show, and developed a great passion for magic. However, due to the Dream Fuel Spirit that Chariot used on the show, Diana lost her ability to use magic. However, she did not give up and tried daily to regain her skills, which she finally achieved.
While growing up, Diana had to face the expectations of being the heiress of the House of Cavendish, which led her to abandon her dreams as a child and put aside her admiration for Shiny Chariot. However, she still decided to attend Luna Nova Academy because it was the school Chariot attended.
Years later, Diana attends Luna Nova but mocks Akko for liking Shiny Chariot. She demonstrates her magic skill by vaporizing a dungeon monster.
In her arrogance, she opens a cursed iron maiden and releases an ancient dragon that grows stronger by devouring magic spells. After being humbled, she attempts to evacuate the other students. She later saves Akko from falling to her death by catching her. Akko suspects Diana knows more about Chariot then she lets on, which she hurriedly denies.
Diana saves a classroom from being devoured by a mushroom monster, created by Sucy and Akko's ingredients, with a nullification spell. Diana continues her cold demeanor and mocks Akko's parade plans. She later gloats about how Akko's plan fell through after Sucy and Lotte abandon her, but unwittingly gives Akko the idea to use the Shiny Rod as a magic battery for the parade.
She later works together with Akko and Amanda to keep the Shiny Rod out of the Titan's hands, expressing relief that Akko is safe and gives begrudging approval of Akko's performance.