Mariel is a human girl residing in a large mansion near the forest where Memole and other inhabitants from the planet Riruru crash-landed. Frail health confines her indoors, fostering profound sadness and isolation before meeting Memole. Her mother is deceased, and her father remains frequently absent due to business, communicating only through occasional letters and gifts. She lives primarily under the care of a strict housekeeper named Penelope. Mariel finds solace in piano playing, demonstrating significant musical talent and a deep emotional connection to the instrument.

Her life transforms upon encountering Memole, a tiny alien girl Mariel initially mistakes for a doll. A pivotal moment occurs when Memole’s tears fall on Mariel’s lips, miraculously reviving her from a critical health decline and forging an inseparable bond. This friendship catalyzes Mariel’s gradual improvement in physical health and emotional well-being. She gains courage to venture outdoors more often, facilitated by Memole’s assistance or animal companions like an owl named Bo-bo for transportation.

As the series progresses, Mariel enrolls in an Art Academy to study music. There, she develops romantic feelings for Oscar, a boy who shares her fascination with aliens. This complicates her social dynamics, particularly with Oscar’s childhood friend Grace—engaged to him in an arranged marriage—who bullies Mariel out of jealousy. Despite Grace’s antagonism, Mariel intervenes to help her after a severe accident, leading to genuine reconciliation and friendship. Mariel also befriends Cynthia, a classmate aspiring to be a ballerina.

Mariel navigates complex emotional challenges, including encouraging Oscar to reconcile with Grace despite her own feelings. Her relationship with Memole deepens to mutual dependence; Memole postpones returning to Riruru when Mariel’s health deteriorates upon learning of the planned departure. Mariel evolves from a lonely, resigned girl into someone capable of forming meaningful relationships, advocating for others, and finding joy through connection.

In the OVA "Marielle no Hōsekibako" and the condensed movie "Wee Wendy," her core characterization and bond with Memole remain consistent with the television series, focusing on their adventures. Official merchandise, such as figures, depicts her as a blonde girl typically dressed in blue. No significant additional background or development occurs in spin-offs or other extended media beyond the original series, OVA, and film adaptations.