Diana, whose full name is Dian Rubens, is a minor but memorable character introduced in the second season of the anime. She resides in the alchemists' quarter of the trading town of Kumerson, a district known for its unpleasant smells resulting from failed experiments. Physically, she is depicted as a woman with a mature and calm demeanor, featuring long dark hair, often styled with a braid, and blue eyes. Her attire typically includes a cape and a necklace, giving her a slightly mysterious appearance that suits her profession and secluded home.

Personality wise, Diana initially gives off an opaque and even somewhat dangerous impression, which is largely a product of her isolated and unconventional living situation. However, once engaged in conversation, she proves to be a kind, perceptive, and surprisingly gentle soul. She is a woman of few words but great insight, showing amusement and understanding when the traveling merchant Lawrence visits her with an unusual request that is less about business and more about personal matters. Her primary motivation in the story is not tied to personal gain or ambition; instead, she appears to live a quiet life, continuing her work as an alchemist while possessing a deep appreciation for ancient tales and legends, which she collects and shares.

Diana's role in the narrative is crucial as a facilitator. When Lawrence finds himself in a desperate financial duel against a rival merchant for the freedom of his companion, the wisewolf Holo, he needs a specific commodity, pyrite, to manipulate the market. Diana is an alchemist who has access to this pyrite. Lawrence seeks her out, and despite the late hour and the strange nature of his request, she agrees to help him. She reveals that her stock has already been sold on credit, but she offers to intercede with the buyer, handling the negotiations confidentially to protect Lawrence's interests. Her home becomes a quiet sanctuary where Lawrence can confess his fears not as a merchant, but as a man fighting for a relationship, to which she listens without judgment. She confirms that she knows of stories where humans and non-human deities form couples, offering him a sliver of hope.

Her key relationships are brief but significant. With Lawrence, she acts as a confidant and a catalyst. She is not an ally in the trenches of the market but a source of essential resources and moral support. Her relationship with Holo is more profound, though it occurs largely off-screen. It is revealed that Holo, who has been secretly working to aid Lawrence, personally visits Diana to purchase pyrite, demonstrating a level of trust and understanding between the two women. Holo also knows Diana's true nature, revealing that Diana is not human but a bird capable of taking a woman's form. Holo recounts Diana's sad history, explaining that she was once the companion of a priest, but was ultimately abandoned by him. This backstory adds a layer of tragic depth to Diana's quiet life and parallels the central theme of the series concerning the difficulties of relationships between immortal beings and mortals.

While Diana does not undergo personal development on screen, her function in the story is to enable the growth of the main characters. By providing Lawrence with the means and the emotional space to confront his feelings, and by affirming the possibility of his relationship with Holo, she helps steer him toward a more honest understanding of himself. Her own tragic past with a priest serves as a narrative lesson, illustrating the potential sorrow that Lawrence and Holo are trying to avoid.

Her notable ability is her knowledge of alchemy, specifically her access to refined metals like pyrite, which places her at the heart of the economic conflict. More significantly, she is revealed to possess the supernatural ability to transform from a bird into a human woman, a secret she shares with Holo. This ability connects her to the older, more magical world of folklore that exists just beneath the surface of the series' otherwise grounded mercantile setting.