Memoru comes from the planet Riruru, arriving on Earth when her spaceship crashes with 244 fellow inhabitants due to mechanical failure. As the granddaughter of the Riruru village chief, she lives on Earth under her grandfather's care while her parents stay on their home planet; she sometimes longs for them.

Tiny enough to fit in a human palm, Memoru has distinctive purple hair and always wears a pointed red hat and red shoes. Her personality is adventurous, fearless, and endlessly curious, setting her apart from the generally cautious Riruru citizens. She forms a deep bond with Mariel, a lonely, seriously ill human girl nearby. Their friendship begins when Memoru's tears fall on Mariel's lips during a critical health episode, miraculously reviving her and forging an enduring connection. Memoru frequently visits Mariel using a magical bubble umbrella to summon her owl companion, Bo-bo, for transport.

Memoru communicates with woodland animals, enlisting their help for transportation or tasks. Beyond Mariel, her close friendships include fellow Riruru children: the adventurous Popit, whom she relies upon, and Lupang alongside his younger brother Pea. She also respects the wandering village elder Ryukkuman for his guidance. These relationships lead to key developments, such as Memoru eventually trusting the humans Oscar and Grace with the secret of her people after initial conflicts.

Her journey shifts from a spirited explorer seeking joyful escapades with Mariel to grappling with responsibility and sacrifice. She confronts the profound interdependence between them, recognizing Mariel's health relies on her presence. This understanding forces a final choice: as the Riruru prepare to depart Earth, Mariel falls ill again. Memoru chooses to stay indefinitely, prioritizing her friend's survival over returning home.

In the OVA "Marielle no Hōsekibako," Mariel's recollections frame their shared experiences, reinforcing Memoru's role as a source of joy and resilience during Mariel's convalescence.