Haru Yoshioka is a seventeen-year-old high school student and the protagonist of The Cat Returns. She lives with her mother, and her father is not mentioned or present in the story. At the beginning of her journey, Haru is portrayed as a typical teenager dealing with the everyday struggles of adolescence. She is often harried, clumsy, and prone to oversleeping, which regularly makes her late for school. More than just being awkward, she is characterized by a deep-seated insecurity and a lack of confidence in herself, described as a shy, quiet, and unassuming individual who would rather be anyone other than her own "gangly self". This low self-esteem is further highlighted by a schoolgirl crush on a popular boy named Machida, whom she is too timid to approach, especially after learning he already has a girlfriend.

Despite her uncertainty, Haru possesses a strong, innate sense of kindness and compassion. The entire plot is set in motion when she impulsively runs into a busy road, using a lacrosse stick to save a mysterious cat from being hit by a truck. This act of bravery reveals her core motivation: a good heart that acts on instinct, even if she does not feel particularly heroic herself. The cat she saves turns out to be Prince Lune of the Cat Kingdom. In gratitude, the Cat King showers her with a series of bizarre and unwanted "gifts," such as cattails she is allergic to and boxes full of live mice. These well-intentioned but problematic presents quickly escalate when Haru is informed she is to marry the Prince and become a permanent resident of the Cat Kingdom. Haru's primary motivation throughout the story becomes her desperate desire to resist this forced fate, reclaim her autonomy, and return to her normal human life.

Her role in the story is that of a reluctant hero who must learn to advocate for herself. Initially, Haru is an extreme doormat, finding it nearly impossible to firmly refuse the cats' demands; her mumbled, ambiguous reply to the Prince’s marriage proposal is immediately taken as a "yes". Desperate for a way out, she follows a mysterious voice to the Cat Bureau, a small agency run by a living cat figurine known as the Baron, his raven companion Toto, and a fat, cynical cat named Muta. Her relationships with these characters are central to her growth. The Baron, a dashing and composed gentleman, becomes her guide and a source of quiet encouragement. Muta provides cynical humor and reluctant loyalty, while Yuki, a white cat who was saved by a young Haru in the past, acts as a secret ally who repays her kindness by helping her escape the Cat Kingdom.

Haru undergoes a significant development arc that is visually and thematically expressed through her physical transformation. As she spends more time in the Cat Kingdom and starts to enjoy its creature comforts, she begins to transform into a cat, first growing cat ears, a tail, and whiskers. This transformation symbolizes the risk of losing her identity and humanity. To break the spell and return to her true self, she must learn to believe in her own worth. By the film's climax, she finds her inner strength, stands up to the Cat King, and fully reverts to her human form. Reflecting her newfound confidence and self-respect, she even cuts her hair short, a change that symbolizes her personal growth and her decision to stop trying to be someone she is not. At the end of her adventure, she explicitly states that she does not regret her actions, showing that she has come to value her own compassionate spirit. While she possesses no magical abilities of her own, her notable traits are her athleticism with a lacrosse stick and a long-suppressed, latent ability to communicate with cats, which she had not used since early childhood.