Kaya is a young woman from Syrup Village, a peaceful hamlet located on the Gecko Islands in the East Blue. She is the sole heiress to the largest and most affluent mansion in the village, having inherited her family's considerable fortune. Kaya is characterized by her delicate and elegant appearance, with pale skin, large brown eyes, and light blonde hair. Initially, she is seen with short, chin-length hair, but as time passes, she grows it out to a longer style. Her typical attire is simple and neat, often consisting of a plain long dress, which aligns with her modest and unpretentious nature despite her wealth.

Kaya's personality is defined by a gentle, kind, and trusting disposition. She is well-liked by the villagers for her warm and approachable manner. However, her background is marked by tragedy. Following the death of her parents from unknown causes, Kaya fell into a deep depression that manifested as a chronic, vague illness, leaving her bedridden and emotionally fragile. Her sorrow made her withdrawn and prone to worry, creating a vulnerable state that those around her sought to manage.

The most significant person in Kaya's life is Usopp, the village's habitual liar and storyteller. Recognizing her loneliness, Usopp began visiting her daily, climbing a tree outside her window to regale her with outlandish tales of his heroic adventures. Even though Kaya always knew these stories were fabrications, they brought her joy and laughter, becoming the brightest part of her dark days and playing a crucial role in her emotional recovery. This shared history forged a deep bond of gratitude and affection between them. She worries for his safety and keeps a close watch on news of his journey as a member of the Straw Hat Pirates, always able to recognize him in wanted posters despite his disguises.

Kaya’s role in the story centers on the Syrup Village arc, where she becomes the target of a sinister conspiracy. Her trusted butler, Klahadore, is secretly Kuro, a cunning and ruthless pirate captain. Kuro spent three years posing as a loyal servant with the sole intention of murdering Kaya on her eighteenth birthday to claim her inheritance. When Usopp accidentally discovered the plot and tried to warn her, Kaya refused to believe him, having complete faith in her caretaker. She was so convinced of Usopp's deception that she slapped him for insulting Klahadore's loyalty. The truth is finally revealed when her other faithful servant, Merry, is gravely wounded by Kuro. Discovering the betrayal, Kaya demonstrates remarkable inner strength. Despite the emotional collapse of realizing her life was a lie, she confronts Kuro, even aiming a gun at him. Later, when cornered by Kuro's hypnotist companion, Jango, who threatens to kill the three children who follow Usopp, Kaya shows immense courage and selflessness. She threatens to kill herself immediately, knowing that if she dies without signing a will leaving her fortune to Kuro, his entire scheme would fail, thus forcing Jango to spare the children.

Following the defeat of Kuro by Monkey D. Luffy and his crew, Kaya's health and spirit make a full recovery. The traumatic event galvanizes her, giving her a new sense of purpose. As a gesture of profound gratitude for saving her life and exposing the treachery in her home, she gifts the Straw Hat Pirates their first true seafaring vessel, the Going Merry. This act cements her as a crucial early ally to the crew. More importantly, her relationship with Usopp inspires her central long-term motivation. She decides to dedicate herself to studying medicine, with the specific and heartfelt goal of one day being able to treat Usopp's injuries when he returns home from his adventures as a pirate.

In terms of development, Kaya transforms from a passive, sickly, and sorrowful heiress into an active, determined, and compassionate young woman. She overcomes her naivety about the world's dangers and channels her experiences into a constructive path, pursuing the dream of becoming a doctor. Years later, after the Straw Hats' journey has progressed significantly, Kaya is shown to be healthy and diligently studying medicine, surrounded by a large collection of medical texts, with a photo of Usopp in his Sogeking persona placed among them. Her final known appearance is during the Egghead arc, where she is seen reacting to a global broadcast, confirming that she remains alive and well, continuing her studies. Regarding notable abilities, Kaya possesses no combat skills or superhuman powers. Her primary strengths are her intelligence, her resolve, and the moral courage she displayed in life-threatening situations. Her most significant action in the story is her philanthropic one: providing the Going Merry, a vessel that becomes a beloved member of the Straw Hat crew itself.