Kokoro is a pretty teenage girl with long light brown hair, dark brown eyes and she is very skinny as noted by Kippei Katakura.
Kokoro tends to be quiet and keeps to herself. The loss of her mother at an early age affected her deeply and also allows her to sympathize with Yuzuyu Sakashita.
Despite the seemingly cold exterior and aloofness, Kokoro is in fact extremely lonely, although for a long time she denies it. After Mrs. Tokunaga (Kokoro's mother) passes away, she is unable to stop crying every day; this led to Mr. Tokunaga (Kokoro's father) boxing away all of his wife's photos and belongings in an effort to stop his daughter's pain, only to propagate it further as Kokoro saw this as her father not caring for her mother and moving on so quickly. Her father believes that since she has done everything on her own since she was little, she is better off by herself and rents a large apartment for her to live in alone after he remarries. Though Kokoro never really wanted to move out and live alone, she couldn't bring herself to tell her father otherwise.
Kokoro originally hated Kippei Katakura due to his playboy antics and how it seemed like he was able to accept and adapt to almost any situation. But, she secretly started to warm up to him after she sees how gentle and caring he is towards Yuzuyu. Even when Kokoro admitted her feelings toward him, however, she refused to allow him to get any closer to her. This was due to how, whenever she was around him, she was always reminded of how lonely she was. Despite this, Kippei continued to try to help her until eventually he helped her to realize that it was okay to be lonely and that he would always do his best to make her happy where they eventually became a couple and Kokoro moves in with Kippei's family soon after.
At the end of the manga, it is presumed that Kokoro and Kippei got married (which now makes her Kokoro Katakura) and they are still living under the same roof.
- Unnamed Father
- Unnamed Deceased Mother
- Unnamed Step-Mother
- Kippei Katakura (Boyfriend)
- The name Kokoro means "heart, mind, soul" (心).
- Kokoro's surname Tokunaga means "virtue, ethics" (徳) (toku) and "eternal" (永) (naga).