Karin Kurosaki is the daughter of Isshin and Masaki Kurosaki, the younger sister of Ichigo, and the fraternal twin sister of Yuzu. She is characterized by a cynical, sarcastic, and authoritative demeanor, which forms a stark contrast to the soft-spoken and gentle nature of her twin. This tough exterior is a direct result of her mother's death six years before the main story begins. Feeling unable to contribute to the household chores like Yuzu, a young Karin resolved to never cry again, believing that showing her personal troubles would burden her family. Consequently, she became the most hardened and least outwardly emotional member of the family, with her older brother Ichigo noting that years had passed since he last saw her cry. Despite this emotional armor, she is a sensible person who shares an equal concern with Yuzu for Ichigo, often reassuring her sister when their brother is stressed or disappears for long periods. She frequently demonstrates her no-nonsense attitude by physically kicking her boisterous father in the head when his antics go too far, yet she remains helpful around the family clinic, assisting as a nurse.

Karin shares Ichigo's ability to see spirits clearly, but she spends much of the early story in denial about this gift, finding the constant attention from ghosts to be a nuisance. As her spiritual awareness grows, she learns to sense Hollows from great distances, which often manifests as a headache. Despite her dismissive attitude toward the supernatural, she is pragmatic enough to use her abilities for practical purposes. She becomes Karakura Red, a member of Don Kanonji's Karakura Superheroes, and develops a unique fighting style that incorporates her athletic talent for soccer. She can channel her spiritual power into powerful kicks, using a technique informally called the Karin-Style Shot of Death, where she kicks a soccer ball or even the modified soul Kon with enough force to injure weak Hollows.

Her role in the story deepens as she becomes one of the first family members to deduce that Ichigo is a Soul Reaper, driven by a desire to understand why her brother is suffering. When Ichigo loses his powers, Karin's own spiritual power grows significantly in response. She becomes more proactive, frequently visiting Kisuke Urahara's shop to obtain spiritual tools, and she expresses a strong determination to be the one who protects Ichigo now that he is vulnerable. Her key relationships are central to her development. She has a close, teasing bond with her sister Yuzu and a deep, unspoken admiration for her brother Ichigo. She forges a unique connection with Captain Toshiro Hitsugaya when he is stationed in the living world; after he helps her soccer team win a match and saves her from a Hollow, she learns his true identity, though she teasingly continues to mistake him for an elementary school child. She also maintains a secretive alliance with Kisuke Urahara, who refuses to charge her for spiritual items out of gratitude for Ichigo's past help.

Karin's development is marked by a gradual acceptance of her spiritual heritage. After the seventeen-month timeskip, she enters middle school, grows her hair into a short ponytail, and her power elevates to that of a high-spec spiritual medium. In the final arc of the story, she and her family are given a Soul Ticket by the new Captain-Commander, granting them free passage to the Soul Society. Following a family discussion, she decides to temporarily live in the Soul Society during school holidays to attend the Shinigami Academy and learn more about Soul Reapers. Upon graduating, she rejects invitations from other divisions to join the Tenth Division as its Third Seat, choosing to serve directly under Captain Toshiro Hitsugaya, with whom she shares a deep mutual respect and friendship.