Marie Lowenbrau is a young orphan girl who appears in the second half of the story. She is approximately four years old and is first encountered on a remote island where her family was killed during an attack by the Neo-Atlanteans. Nadia and Jean discover her among the wreckage, alive but unconscious, and take responsibility for her care. She subsequently joins the group and becomes a passenger aboard the Nautilus.

Marie is portrayed as remarkably intelligent and charismatic for her age. Despite the trauma she has endured, she displays a resilient and cheerful disposition, often bringing warmth and lightness to the grim situations the group faces. She is trusting and forms attachments quickly, especially toward Nadia and Jean, who become surrogate parental figures for her. Her innocence contrasts with the darker themes of the series, and she serves as a reminder of what is at stake in the conflict against the Neo-Atlanteans.

Her primary motivation is simply to be safe and cared for, and she looks to the older characters for protection and guidance. She does not drive the plot forward but instead acts as a catalyst for emotional depth in the main characters, particularly Nadia, whose protective instincts are brought to the forefront through her interactions with Marie. Marie also becomes a close companion to other members of the group, and she forms a special bond with King, the lion cub, who becomes her playmate.

Over the course of the story, Marie grows and matures, and by the end of the series, she is shown as an adult narrating the conclusion, providing a reflective perspective on the events and the ultimate fate of the main characters. She does not possess any supernatural abilities or special skills; her significance lies in her role as a symbol of the innocence that the heroes are fighting to protect and as a testament to the lasting impact of their journey.