Kokoro
Kokoro is Kippei's classmate. She's a very reserved and serious person who seems to be a great deal more mature than her classmates. She initially has a very low concept of Kippei due to his womanizing ways and apparent lack of seriousness, but in time she grows to appreciate Kippei's company precisely because he doesn't take things too seriously, unlike herself.
Even though she has two loyal friends in Motoki and Aki, Kokoro is a very lonely person. Her dad remarried after her mother died, and her new family doesn't seem to understand her (or care to try to). They assume she prefers to be on her own because she's so independent and reserved, but in reality Kokoro longs for a family to care for her.