Oiseau Bleu originates from the fictional German fairy tale "Liz and the Blue Bird," the narrative foundation for a concert band piece within the story. In this tale, Oiseau Bleu is a blue bird who transforms into a human girl after meeting a solitary woman named Liz. They form a deep bond and live together happily. However, Liz realizes Oiseau Bleu's true nature and potential fulfillment lie beyond their domestic life, leading Liz to set the bird free. This act allows Oiseau Bleu to return to her avian form and live autonomously, symbolizing liberation.

Within the anime's context, Oiseau Bleu functions as a symbolic representation paralleling the relationship between the two main human characters. Musically, she is associated with the flute part in the band piece, characterized by light, airy, and energetic motifs reflecting her cheerful and unrestrained nature, contrasting with Liz's representation through the oboe's grounded and melancholic tones. Oiseau Bleu's story arc—from transformation to separation—serves as a metaphor for interpersonal growth and the necessity of release for mutual flourishing.

The human characters initially misinterpret their parallels to the fairy tale: the reserved oboist identifies as Liz and the outgoing flutist as Oiseau Bleu, reflecting their real-world dynamic where one perceives herself as the anchor and the other as the transient force. Deeper introspection reveals an inverted alignment: the flutist embodies Liz's role in needing to release her friend, while the oboist embodies Oiseau Bleu's journey toward self-reliance after separation. This thematic revelation shifts their disjointed musical performance into harmony once they internalize the fairy tale's core message of loving detachment.

Visual depictions of the fairy tale within the anime employ a distinct pastel, storybook aesthetic contrasting with the main narrative's realistic style. Oiseau Bleu's design emphasizes fluid movement and vibrant blue hues, reinforcing her association with freedom and the natural world. Her joyful interactions with Liz transition to confusion and sorrow during their separation, culminating in a final sequence where Oiseau Bleu, restored to bird form, soars skyward, visually reinforcing the theme of transcendence through release.