Hanon Houshou is the mermaid princess of the South Atlantic Ocean and the guardian of the Aqua Pearl. She is a main character in the story, first appearing as the second mermaid princess to arrive on land and quickly becoming a close companion to the series' protagonist, Lucia. Her decision to leave the sea was not driven by romance but by necessity, as she fled to the human world to escape water demons who had attacked and destroyed her kingdom, forcing its inhabitants to flee.

In terms of appearance, Hanon has different looks depending on her form. As a human, she has shorter, darker azure hair and brown eyes, often wearing flower-shaped barrettes crossed like an X in her bangs. In her natural mermaid form, her hair is long, wavy, and blue with two star-shaped ornaments, and her eyes become aqua blue. Her mermaid attire includes an aquamarine clamshell bra and a darker blue tail. She is the second shortest of the mermaid princesses, with her human form standing at approximately 152 to 153 centimeters tall.

Hanon's personality is characterized by a playful, sociable, and friendly nature. She is very feminine, enjoys shopping, and has a talent for makeup, which are among her primary hobbies. Beneath her cheerful and sometimes flirty exterior, she is also astute and observant, displaying a profound sense of responsibility that befits her status as a princess. She often considers herself mature and worldly, especially regarding matters of the heart, and freely offers advice to her friends about love. Despite this self-perceived wisdom, she can be hot-headed, stubborn, and prone to jealousy when it comes to her own romantic interests, sometimes acting impulsively and without regard for her personal safety.

Her primary motivation evolves throughout the narrative. Initially, finding safety on land is her main goal, but this quickly shifts as she forms bonds with others. Her most significant early motivation revolves around her first love, her music teacher, Taro Mitsuki, whom she affectionately calls Taro-chan. She is deeply devoted to him and is willing to risk her life for him, which becomes a key driver of her actions in the first major story arc. A core part of her character development is learning to navigate heartbreak. When she discovers that Taro is still in love with a woman from his past named Sara, she is heartbroken but demonstrates maturity by accepting his feelings and even risking her own safety to protect him, an act of sacrifice that contributes to Sara's reformation. Following the events of the first season, Taro leaves for Germany to study music, and just before his departure, he gives her a musical composition he wrote for her.

This loss sets the stage for her development in the second season, Pure. While she initially struggles to move on from Taro, she eventually meets a classmate named Shiroi Nagisa. Nagisa finds the sheet music Taro left for Hanon and quickly develops feelings for her. Hanon is initially dismissive of him, finding his behavior immature, but over time she reciprocates his affection, and they begin a relationship. This journey from an unrequited crush on an older figure to a reciprocal relationship with a peer represents a significant step in her emotional growth.

In the story, Hanon serves as a deuteragonist, forming a core trio with Lucia and Rina. She is instrumental in battles against the primary antagonists, Gaito and later Michel, using her singing voice as a weapon. Her key relationships are central to the plot. Her friendship with Lucia is established from Lucia's first day at school, and Hanon is known for being reliable and for keeping her promises, no matter how difficult. She also forms a strong bond with Rina, rounding out the main group of heroines.

As a mermaid princess, Hanon possesses several notable abilities. Her power is derived from her Aqua Pearl, which she keeps in a shell locket around her neck. She uses the pearl to transform into her idol singer form, known as Mizuiro Pearl Voice, or Aqua Pearl Voice. To initiate this transformation, she calls out "Mizuiro Pearl Voice!". In this idol form, her voice is amplified by a microphone-like device called the E-pitch, and she uses her songs as an offensive attack to defeat enemies. In the anime, she typically concludes her attacks by saying "Love Shower Pitch!", while in the manga, she says "Pretty Sugar Pitch!". Her main image songs, which serve as her special attacks, are Ever Blue and Mizuiro no Senritsu (Aqua Melody). Later in the series, she and the other princesses receive an upgrade from the sea goddess Aqua Regina, granting her a more powerful second idol form with an enhanced appearance and stronger singing abilities.