Diana is a central antagonist in the narrative, holding the title of Queen within the Chess Pieces, a position that makes her the second most powerful member of that organization. She is also the former queen of the Kingdom of Lestava. Her origins trace back to Caldia, the island nation of witches, where she was once a well-liked citizen and a caring older sister to her sibling, Dorothy. During her time in Caldia, she was known to have trained Dorothy in the use of wind-based magical weapons and would create and repair dolls for her, one of which would later become the basis for the Knight Pinocchio.

Diana’s personality is defined by a dramatic shift from her past self. For much of the story, she is presented as sinister, selfish, and calculating, preferring to remain in the background while commanding others to do her bidding. She is the force who pulls the strings of the Knights, using them as weapons in her plan to conquer both MÄR-Heaven and the world of Ginta Toramizu. Her ambition is driven by an immense craving for power, which motivated her to betray her homeland of Caldia a decade before the series begins. Alongside a warrior named Phantom, she released a powerful entity and stole nearly eight hundred magical items known as ÄRMs, including a sentient one named Babbo. However, it is revealed that her true, more loving and gentle self was locked away and clouded by the influence and manipulation of the Chess Pieces' King. This authentic personality can only surface at the moment of her death.

Her role in the story is as a primary instigator of the conflict. She transformed Lestava Castle into a safe haven for the Chess Pieces and shaped the land into a state of war. Her ultimate goal is to dominate Ginta's world, believing she can use its advanced technology and weapons to solidify her power over MÄR-Heaven. To this end, she creates the Cross Guard and orchestrates key events, including the capture of her step-daughter, Snow, whose connection to a girl in Ginta's world is essential for opening a portal between the two dimensions.

The most significant relationship in Diana's life is with her younger sister, Dorothy. While Dorothy respects Diana's power and was once close to her, the two become bitter enemies due to Diana's turn to evil. Diana also becomes the step-mother to Snow after marrying Snow's father, the King of Lestava, and initially bonds with her instantly. She later manipulates Snow as a tool for her plans. Her relationship with Phantom is that of a commander to a favored weapon, rewarding him with immortality but eventually discarding him when he fails her.

Diana undergoes her most critical character development at the end of her arc. Following the fall of Lestava Castle, she seizes Caldia's floating castle as a new base. In the final confrontation, she chooses to battle Dorothy. The fight ends with Dorothy delivering a fatal wound, and in her final moments, Diana returns to her true self as a loving sister. She apologizes to Dorothy and dies in her arms, bringing a measure of sorrow to their reconciliation.

In terms of abilities, Diana is exceptionally powerful, considered one of the strongest characters and the most powerful witch in MÄR-Heaven. She wields a vast collection of ÄRMs, each with unique functions. Her signature weapon is the Aeros Staff, a scepter that grants her mastery over wind. She possesses the Ghost ÄRM Dalia, which transforms her into a harpy-like creature with wings and claws. Her Guardian ÄRMs include a giant octopus, a giant construct named Pinocchio II, and Cerberus, a multi-headed monster summoned via her choker. She also uses a Nature ÄRM called the Ring of Ends, which creates a poisonous mist, and in the anime, she employs the Death Cube, a Darkness ÄRM that traps a person in a puzzle-like prison. Crucially, she owns the World Tunneler, a unique ÄRM that appears as a large bubble, capable of using Snow's connection to the other world to open a portal between dimensions.