Maia Mizuki is the central protagonist of the series. She is a fifteen-year-old orphan who, at the story's outset, has just graduated from middle school. Her life took a tragic turn in early childhood when she was involved in a car accident caused by a control malfunction. The crash resulted in the death of both of her parents and put Maia into a coma for one year. When she awoke, she had lost all of her childhood memories and was introduced to a man who claimed to be her grandfather, who subsequently raised her. This grandfather, whose final word to her was daphne, died shortly before the events of the first episode.

Maia is characterized as an optimistic, hardworking, and honest young woman. She has a shy and naive demeanor, often coming across as the archetypal girl-next-door. Despite her inherent sweetness and moral center, she finds herself frequently thrust into dangerous and absurd situations. A survival mantra taught to her by her grandfather is a recurring touchstone for her character: A tree that is planted by water will produce fruit in due season, its leaves will never wither. She recites this verse during moments of extreme difficulty or danger, reflecting her deep-seated need for hope and stability.

The primary motivating force behind Maia's actions is the search for her lost past. She is driven to uncover the secrets of her childhood and the truth about her family, a mystery that becomes increasingly complex as the series progresses. This quest is directly tied to her immediate circumstances in the story. Despite being an excellent student, Maia is inexplicably rejected from the elite Ocean Agency, a futuristic paramilitary organization. This rejection, combined with the recent death of her grandfather, leaves her homeless and penniless after she is evicted and pickpocketed. Following a chaotic hostage situation in which she is shot, Maia is rescued by and subsequently joins Nereids, an unorthodox help-for-hire agency. Her role in the narrative is that of the newest member of this group, where she often serves as the moral compass, or the heart, of the team, frequently objecting to her coworkers more extreme or unethical tactics.

Her key relationships are primarily with the other members of Nereids, including Rena, Shizuka, and later Gloria and Yu. While her co-workers are often rude and volatile, Maia develops a sense of belonging and finds satisfaction in her work despite the life-threatening danger. The most significant figure in her backstory is her grandfather, whose true identity and hidden secrets form the core of the series central mystery. It is eventually revealed that the man who raised her was not her biological grandfather but rather her older brother, who was awakened from a hibernation pod decades before she was.

Throughout the series, Maia undergoes notable development. Initially, she is a classic damsel in distress, frequently ending up bound, gagged, or taken hostage to the point that it becomes a running gag. However, she gradually takes a level in badass, evolving into a more confident and capable action girl. Early episodes show her fumbling with a gun and being easily subdued, but later installments portray her as someone who can handle a firearm with confidence, set sophisticated alarms, and take enemies by surprise.

In terms of notable abilities, Maia is academically gifted, having passed the grueling Ocean Agency qualifying exam with flying colors. She also possesses an uncommon hovercraft operator's license, which she obtained because it was required for her job examination, a skill that proves vital on several occasions. Her quick wit and ingenuity in the field are also highlighted as key assets that allow her to contribute to the Nereids missions despite her initial lack of combat experience.