Kumi is Ryo Mashiba's sweet-natured, but hard-working and willful younger sister, and as such understands him better than anyone else. She will do almost anything to support her brother and make him happy, including going to his matches even though she doesn't like to see him get hurt. Ippo becomes infatuated with her early on.
But didn't have a chance to tell her until more than a year later, when they met in an hospital as patient and nurse some months after the Ippo/Mashiba ring. They become friends and Kumi comes to like Ippo back, but their relationship tends to suffer from their friends' and relatives' constant interference.
