Marie is a mermaid Xenos first encountered by adventurer Bell Cranel in the Water City sector on the 25th floor of the Dungeon. She possesses emerald-blue hair, jade-colored eyes, fins instead of ears, and faint indigo-blue skin on her upper body, complemented by a fin matching her hair color. Following her initial encounter with Bell, she began using shells to cover her breasts. Exhibiting innocence and a childlike demeanor, Marie communicates with limited skill, conveying an impression of immaturity. Despite this, she adeptly avoids hostile monsters and adventurers within the Dungeon. Her personality blends deep care for fellow Xenos with pronounced wariness of stronger monsters, often reacting with terror due to her lack of combat abilities. She displays inherent kindness, notably through her willingness to use her own blood—which possesses potent healing properties capable of reattaching severed limbs—to aid others without hesitation. Her development centers around her relationship with Bell. After he protected her from the Moss Huge and fulfilled his promise to defeat it, Marie developed romantic feelings for him. These feelings later motivated her to overcome intense fear during the Juggernaut's emergence. Initially fleeing into the water, she resolved to search for Bell upon recalling her feelings, discovering his severely injured body. She then deliberately cut herself with his knife to enhance blood absorption, facilitating his recovery. As a Xenos, Marie possesses heightened intelligence and capabilities surpassing ordinary monsters. Her primary abilities include exceptional swimming speed surpassing Level 4 adventurers and enabling evasion of aquatic threats. She also utilizes a charm ability through song, effective against both adventurers and weaker monsters, which she employed to subdue creatures aiding Bell's navigation through the Dungeon.