Catherine is a character whose background and true identity are intentionally shrouded in mystery. The name Catherine is believed to be an alias, and even the protagonist, Natsuki, does not know her real name or her true origins. Her primary reason for being in the town is to collect a debt, as she possesses a promissory note from Natsuki's late grandmother. However, she presents a proposition to Natsuki: rather than simply collecting the debt, they should work together to salvage a valuable inheritance that his grandmother, a marine geologist, left hidden in an underwater warehouse. She proposes they split the profits evenly. Her knowledge of this hidden treasure and her connection to Natsuki's grandmother are part of her enigmatic nature.

In terms of personality, Catherine initially comes across as a pragmatic and somewhat unscrupulous individual. She is often motivated by money and is not shy about expressing her commercial instincts. When she first encounters the robot Atri, her immediate reaction is to recognize her potential monetary value, enthusiastically commenting on the high price she could fetch. This attitude paints her as a potential antagonist or, at the very least, an untrustworthy ally. However, this is not her only facet. Catherine struggles with heavy drinking, frequently getting drunk alone in the town and passing out in the streets, only to be found and taken care of by the younger characters, Natsuki and Ririka. This behavior hints at a troubled past and a deep-seated unhappiness beneath her tough exterior.

Catherine's role in the story evolves significantly. Initially, her function is that of a catalyst, pushing Natsuki to dive for the treasure, which leads to the discovery of Atri. She is a constant presence, often acting as a foil to Natsuki’s growing emotional attachment to Atri by reminding him of their original financial goal. As her backstory is revealed, her motivations become clearer. It is disclosed that she was once an elementary school teacher in the same town. She lost her job when the school was submerged due to the rising sea levels and subsequently left. Later, working as a private detective in the city, she was deceived by a man involved with the criminal underworld, who swindled her out of all her money and left her with the debt note that brought her back to the town.

This revelation reframes her character from a simple con artist into someone who has experienced significant loss and betrayal. Despite her personal struggles, Catherine's skills as an educator are genuine. She is shown to be highly capable with children, has a talent for calming and managing them, and can even play the piano. Later in the story, she accepts Natsuki's invitation to return to her former profession, becoming a teacher at the local school for the remaining children, including Natsuki, Atri, and their friends. Her effectiveness as a teacher is demonstrated by the fact that, thanks to her dedication, three students from that small, struggling town eventually go on to be admitted to a research institute.

Her key relationships are primarily with Natsuki and Atri. Her relationship with Natsuki starts as a tense business partnership, but evolves into a more trusting alliance as they face challenges together. With Atri, her arc is one of conflict and eventual respect. When she learns the truth about Atri's past and the potential danger she might pose, Catherine initially acts to protect her students by trying to have Atri banned from school. However, once the situation is resolved, she shows her integrity by offering Atri a sincere, if somewhat awkward, apology. While she ultimately cannot bring herself to attend Atri's farewell party, the apology itself is a significant moment of growth. Her journey is from a cynical, money-driven drifter to a dedicated educator and a flawed but ultimately caring protector of the young people in her charge.