Kyouko Sasagawa is a student at Namimori Middle School and the younger sister of Ryohei Sasagawa, the Vongola Family's Sun Guardian. Within the story, she is primarily known as the object of protagonist Tsunayoshi Sawada's deep affection and admiration, often serving as a key source of motivation for him. Physically, Kyouko is depicted as a slender, short-statured teenage girl with large, bright golden eyes. Her hair is a golden-brown shade, typically cut to reach the back of her neck with longer bangs, though she later sports a shorter style. Her standard attire consists of the Namimori Middle School uniform: a white short-sleeved shirt, a dark sweater vest, a red bowtie, a gray skirt, black knee-high socks, and brown shoes. In the future storyline, her appearance changes to include much longer hair, a dark green hooded jacket worn over a light green knee-length dress, and beige boots.

Kyouko's personality blends natural intelligence with a notably carefree and air-headed demeanor. She tends to accept the many bizarre and dangerous events occurring around her without deep questioning, often dismissing them as school activities or jokes. This includes readily believing her brother's outlandish explanations for his frequent injuries, such as claiming a sumo wrestling competition or a falling star as the cause. Fundamentally, she is an intrinsically kind and caring individual who consistently looks out for her older brother and friends. She rarely displays anger, but does so on principle when treated as an object, such as when a classmate called her a "prize" in a competition, or when she is persistently kept in the dark about dangerous truths. In the latter instance, she staged a temporary strike, refusing to cook or clean until the boys explained the situation involving the Millefiore Famiglia.

Her motivations are largely driven by concern for those close to her. A formative childhood incident, where she was taken hostage by older children to lure her brother into a trap, resulted in him receiving the scar on his head and left her with a strong aversion to violence and fighting. She worries immensely whenever her brother gets injured. Despite this, she provides unwavering, if often unaware, support to Tsuna and his friends. Her presence serves as an emotional anchor for the group, representing a peaceful, normal life worth protecting. Her key relationships are central to her character. With her brother Ryohei, she shares a protective and loving bond; he is her prime motivation, and she is often the reason he fights, just as he is determined to keep the violent side of his life a secret from her. Her feelings for Tsuna are hinted at but never explicitly defined, though she grows to trust him implicitly. She also develops a close friendship with the energetic Haru Miura, with whom she shares a monthly tradition of eating as many cakes as they want, a day Haru named "Haru Appreciation Day."

Throughout the series, Kyouko shows notable development, particularly during the future arc. When she and Haru are sent ten years into the future and held in a Vongola base for their safety, she becomes frustrated with being sheltered and demands to know the truth about the mafia, a world she had only glimpsed through her brother's injuries. Upon learning the truth, she chooses to stay and help rather than return home, providing crucial moral support and even helping Tsuna by encouraging him to open his box weapon. Regarding abilities, Kyouko possesses no superhuman powers or special flames like the main fighters. Her brother taught her how to box following the hostage incident, but she is not shown to be particularly skilled and rarely uses this knowledge. She is, however, one of the few characters to have been shot with Reborn's Dying Will Bullet, which on two occasions forced her into a quiet, polite, and unnervingly destructive state.