Yuna is the Mother Unit in charge of the systems on Terra. After finding a dying carbon, she used parts of her shell, or body, to heal the carbon but ended up having to take over the carbon's body when she found her shell could no longer function. The carbon happened to be Roll's mother, Matilda.

Stuck in the body of Matilda, Yuna delivers a warning to those in the Sulpher Bottom who seek the Mother Lode on Forbidden Island. Despite them ignoring her warning, she takes the side of the carbons in the struggle against Sera.

Although using Matilda's body, all emotions, characteristics and the voice heard are those of Yuna. She is one of the dominant characters of the story, even though her own actual form remains lifeless for the most part.