Kokoro Tokunaga is a quiet and pretty teenage girl with long light brown hair and dark brown eyes. She attends the same high school as the main protagonist, Kippei Katakura. Kokoro's background is marked by early loss and loneliness. Her mother passed away when she was six years old, a tragedy that defines her emotional landscape and allows her to deeply empathize with others who have experienced similar abandonment, particularly the young Yuzuyu Sakashita. After her mother's death, her father remarried and, believing his daughter to be self-sufficient, provided her with a luxurious apartment to live in alone. This left Kokoro feeling isolated and fending for herself, belying the assumption of independence placed upon her. In terms of personality, Kokoro is initially reserved and lonesome, keeping to herself at school. She initially harbors a strong dislike for Kippei due to his reputation as a thoughtless playboy. However, as she witnesses Kippei's genuine tenderness and dedication in taking care of his five-year-old cousin Yuzuyu, her feelings slowly transform into affection and respect. She is understanding of Kippei's demanding responsibilities, showing patience and maturity for her age. Her key relationships are central to the story. Her bond with Kippei develops from animosity into a deep, mutual love as she becomes his primary support system, offering him comfort and assistance with Yuzuyu. Her connection with Yuzuyu is particularly significant; Kokoro sees her younger self in the little girl, and this shared experience of maternal loss forges a strong, nurturing relationship between them. She also maintains a close friendship with her classmates Aki Kagami and Mai Motoki. Throughout the series, Kokoro undergoes significant development. She moves from a state of passive loneliness to actively building a family-like unit with Kippei and Yuzuyu. She learns to open her heart to others, overcoming the emotional barriers built by her father's neglect. Her quiet strength and unwavering loyalty make her an essential part of Kippei's life as he learns the true meaning of responsibility. Kokoro's greatest ability is her profound emotional intelligence and empathy. She possesses an innate ability to understand the pain and needs of Yuzuyu, often acting as a bridge between the child and Kippei's sometimes clumsy attempts at parenting. Her calm and steadying presence provides stability for the small family they create together.