Marcille Donato is a half-elf mage and a central member of the adventuring party that descends into the dungeon. She is fifty years old at the start of the story, born on April 19, and her mixed heritage shapes much of her identity: her mother was an elf, while her father was a tall-man, or human, who passed away when Marcille was still young. She possesses long blonde hair, green eyes, and the slightly rounded ears that signal her half-elf status. A graduate of the Magic Academy and one of its most talented students, she joined a research program specializing in dungeon creation, and it was there that she met Falin Touden, who became her closest friend. Marcille wields the Ambrosia Staff, a magical instrument woven from a branch of the World Tree.

Personality wise, Marcille is intelligent, principled, and deeply loyal to her companions. At the same time, she is high-strung and easily unsettled by situations far outside her comfort zone, most notably the concept of eating monsters, which initially fills her with disgust and anxiety. She has a great deal of confidence in her magical abilities but can feel insecure and put herself at risk to prove her worth when she is unable to contribute effectively. She can also be somewhat naive, believing that friendship alone is enough to hold a party together. Underneath her sometimes dramatic reactions, she is kind-hearted and emotional, carrying a deep-seated fear of losing those she loves because of the long lifespan she inherited from her elven side. This fear, born from watching her human father age and die while she barely changed, drives her primary motivation: to find a way to make all races live equally long so that she will not have to outlive her friends. This desire leads her to study forbidden ancient magic, a dangerous discipline that draws power directly from the dungeon's infinite mana source.

Within the story, Marcille serves as the party's mage, specializing in earth and life magic, healing spells, and defensive barriers. Her role initially is that of the conventional, risk-averse member who wants to rescue Falin using standard methods and return to normalcy. As the journey progresses, she is forced to adapt and learns to appreciate the dungeon as an ecosystem, slowly shedding her rigid prejudices. Her most drastic act occurs when she uses forbidden ancient magic to reconstruct Falin's partially digested body using dragon flesh, an act that succeeds in resurrecting her friend but also creates unintended consequences. This moment marks a turning point in her development, transforming her from a cautious, by-the-book mage into a powerful and morally complicated sorceress willing to cross boundaries for the sake of those she cares about. By the end of the series, she has grown into Laios's most trusted advisor and a key figure in the new order that emerges.

Her key relationships include the deep bond with Falin Touden, which is the emotional anchor of her journey. She is loyal to Laios Touden, the party leader, even as she often reacts with exasperation to his eccentric fascination with monsters. With Senshi, the dwarf who cooks those monsters, Marcille shares a philosophical clash: she sees the dungeon as a resource and a system to be studied and improved, while he views it as an ecosystem to be respected and balanced. This tension frames an ethical question at the heart of the series about whether good intentions justify rewriting nature.