Ouna is the adoptive mother of Princess Kaguya, found by her husband, Sanuki no Miyatsuko (Okina), as a miniature girl within a glowing bamboo shoot. She actively helps raise the child, insisting from the start that the foundling receive proper care and attention, reflecting her practical, nurturing manner.

When Okina discovers gold and fine fabrics in the bamboo grove, interpreting them as divine signs of Kaguya's noble destiny, he moves the family to the capital against Ouna's protests. She disagrees immediately, recognizing the move conflicts with Kaguya's happiness and simpler rural upbringing. However, Ouna lacks authority to stop Okina's plans, revealing her limited agency within the family despite her insights.

In the capital, Ouna maintains her grounded nature despite their elevated status and wealth. She continues household tasks, often retreating to gardens or sewing rooms—spaces she sometimes shares with Kaguya. These areas become sanctuaries where Ouna provides emotional stability and comfort, especially during Kaguya's distress over noble life's restrictive expectations, suitors, and etiquette training.

Ouna demonstrates a deeper understanding of Kaguya's emotional struggles compared to Okina. She witnesses Kaguya's profound unhappiness with the enforced aristocratic lifestyle and recognizes her daughter's longing for freedom and her past rural life. While Okina focuses on ambitions for Kaguya's societal advancement, Ouna offers quiet, consistent empathy. Her support is evident when she facilitates Kaguya's visit to see cherry blossoms bloom outside the capital, acknowledging her need for nature and simpler pleasures.

As celestial beings arrive to reclaim Kaguya for the Moon, Ouna shares the family's profound grief. She is present during Kaguya's final moments on Earth, participating in the emotional farewell as Kaguya dons the celestial robe that erases her earthly memories. Ouna's anguish underscores the deep maternal bond formed throughout their time together.