Kokoro Tokunaga is a teenage girl attending the same high school as Kippei Katakura. She has long light brown hair and dark brown eyes, and she is noted to be quite slender. Her demeanor is quiet and reserved; she tends to keep to herself rather than seek attention. Kokoro lost her mother at the age of six, an early loss that left her with a deep sense of loneliness and shaped her ability to understand the pain of others. This painful experience makes it natural for her to sympathize with Yuzuyu Sakashita, the five-year-old cousin whom Kippei is suddenly tasked with caring for.

After her mother’s death, Kokoro’s father remarried and, believing she could manage on her own, provided her with a luxurious apartment where she lived alone. In reality, she struggles with isolation and yearns for meaningful connection. This independence, however, is paired with a strong sense of responsibility. She does not rely on others easily and often tries to bear her burdens quietly.

At first, Kokoro is put off by Kippei’s reputation as a playboy and keeps her distance. As she observes his genuine dedication to Yuzuyu, her opinion softens and she gradually opens up to him. She becomes his love interest and later his girlfriend, and she is remarkably understanding of the time and attention he must devote to his young cousin. Her patience and loyalty are steadfast; even when Kippei acts foolishly or others try to win his attention, Kokoro’s feelings remain true.

Kokoro plays a pivotal role in the story as a stabilizing presence for both Kippei and Yuzuyu. She often helps with Yuzuyu’s care, drawing on her own childhood memories to offer comfort and guidance. Her quiet strength and empathy encourage Kippei to grow more responsible and emotionally mature. Over time, her own loneliness is eased as she becomes part of Kippei’s family, eventually moving in with them when they recognize how isolated she has been.

Her close friendships with Aki Kagami and Mai Motoki provide a support network at school, but it is her relationship with Kippei and her bond with Yuzuyu that most deeply affect her development. Through caring for Yuzuyu and accepting care from Kippei’s family, Kokoro learns to trust and lean on others, overcoming her fear of abandonment. Her notable ability is her profound emotional sensitivity, which allows her to read the feelings of those around her and respond with genuine kindness, even when she is hurting herself. This empathy makes her an essential anchor in the lives of the people she comes to love.