Mikaru Amagi is a wealthy but deeply lonely girl who comes from a family with ties to the Panthalassa Clan, an ancient sea-based group. She is the other half of the being known as Michel, and she is the younger sister of Rihito. As a result of her father's experiments, she possesses Panthalassan blood, and her physical condition was deliberately weakened to serve a purpose connected to the clan. This weakness manifests as a chronic sickliness, causing her to collapse at times when Michel appears nearby or when Kaito draws close to Lucia, often as a way to keep Kaito's attention on herself. She is also described as someone who has lost all hope before the events of the story.

Mikaru's primary role in the story begins in the second season, Mermaid Melody Pichi Pichi Pitch Pure, when she discovers Kaito Domoto on a beach after he has lost his memory following a surfing accident. She takes him in and develops an intense, one-sided romantic attachment to him. Her feelings become so consuming that from Lucia's perspective, it seems Kaito has completely forgotten his love for the mermaid who once saved him. Mikaru is desperate for Kaito to return her affections and does everything in her power to keep him by her side, including feigning illness or emotional outbursts to prevent him from leaving her or reuniting with Lucia.

Her personality is a mixture of kindness and manipulative behavior. When not driven by her obsession, she can show genuine care, but her loneliness and fear of abandonment push her to act in possessive and sometimes deceitful ways. In the original manga, this desperation reaches a peak when Kaito explicitly tells her he loves only Lucia and could never love her the way she loves him. In response, Mikaru offers herself to him in a last-ditch attempt to keep him, but Kaito turns away and leaves. This rejection triggers a severe physical collapse that allows Michel to fully take control of her body.

Throughout the series, Mikaru functions initially as an antagonist who interferes with the relationship between Lucia and Kaito, and by extension, obstructs the mermaid princesses' mission. Later, she shifts into a more sympathetic supporting role after her true nature and tragic background are revealed. Her development culminates in her being reborn as a pure, blank-slate child who is entrusted to her brother Rihito to raise from scratch, giving her a second chance at life free from the trauma and manipulation she endured.

Mikaru's notable abilities are not those of a combatant or a mermaid princess. Her significance lies in her connection to Michel and her weakened constitution, which serves as a vessel for Michel's will. She does not wield any special powers of her own, but her Panthalassan heritage makes her a key piece in the conflict between the mermaid princesses and the forces threatening the ocean. Her emotional fragility and physical weakness are central to her character, shaping every action she takes and every interaction she has with others.