Marielle Legrand is a fourteen-year-old human girl who serves as the deuteragonist of Tongari Bōshi no Memoru. She comes from a wealthy family, but her mother has passed away and her father is almost entirely absent due to work. Because of her frail health, she is sent to a remote mountain lodge near the border of Switzerland and France to convalesce. There she lives under the strict supervision of her tutor and chaperone, Penelope, who forbids her from going outside and restricts her contact with others, including her own father over the telephone. As a result, Marielle has grown up with almost no friends or meaningful human interaction; her only companion before the story begins is her cat, Jojo.

Marielle is naturally introverted, timid, and deeply lonely. She is polite and well‑mannered, but her isolation has left her with a fragile sense of self‑worth. She often expresses a desire not to be helped, even after being saved from the brink of death. Her great passion is playing the piano, and she is a talented pianist enrolled in the San Roanne Art Academy, though her illness prevents her from attending regularly. Her music becomes a key means of emotional expression and later a way to communicate with her friend.

Her motivation throughout the series is to find connection and meaning in a life that feels empty. She initially despairs, feeling that her existence is a burden, but everything changes when she meets Memoru, a tiny, energetic alien girl from the planet Riruru. Memoru’s reckless kindness and unwavering friendship gradually draw Marielle out of her shell. The two become inseparable, and Marielle begins to rediscover the joy of living. She starts to venture outdoors, explores nature, and eventually returns to school life at San Roanne, where she faces bullying from a wealthy rival named Grace but also makes new allies, including the kind‑hearted boy Oscar and the shy dancer Cynthia.

Marielle’s development is marked by a slow but steady growth in confidence and physical strength. Her health improves as her emotional state brightens. She learns to stand up for herself and to trust others. In a pivotal episode, she and Memoru become separated during a storm, and Marielle plays her piano with all her might so that the sound can guide Memoru back to her. This act demonstrates her deepened resolve and the strength of their bond. By the end of the series, she chooses to remain on Earth with Memoru rather than return to her old life of comfort and solitude.

Her most notable ability is her piano playing, which she uses not only as a creative outlet but also as a means of reaching Memoru across distances. She has no supernatural powers, but her empathy and quiet determination are her greatest strengths. Her key relationships are with Memoru, who serves as her catalyst for change; with Oscar, a sympathetic boy who becomes a close friend and possible love interest; and with her cat Jojo, who provides comfort during her loneliest moments. Her relationship with Penelope evolves from one of strict control to mutual respect as Penelope begins to understand Marielle’s need for freedom. Marielle’s arc is a classic tale of overcoming isolation and illness through the healing power of friendship.