BackgroundWhen Kumi was still in high school, her parents died in an accident. Kumi and Ryou were over their parents graves, Kumi was crying repeating,' What are we going to do?'. Ryou started his oaths.
Her first appearance in Hajime no Ippo, she was walking with a friend to school. During Hajime no Ippo, she and Ippo have had many reunions. Before Mashiba Ryou lost the Rookie Tournament, she worked in a bakery. Ippo had seen her at a match and assumed she was Miyata's fan. He would go very often to the bakery to see her and buy some bread, developing a little crush on her, but he never spoke to her much. She is then seen looking very tense and sad during the match in which Miyata lost. Ippo interprets this as her feeling sad for her idol Miyata losing, and attempts to comfort her when he sees her in the bakery. It is here she reveals, upon snapping at Ippo, that she is in fact Mashiba Ryou's little sister, and this is why she appears in all those matches. Takamura, Aoki, and Kimura went to tease Ippo by going to the bakery upon hearing of this sudden twist, but Ippo himself does not see her anymore due to the awkwardness of the situation. During his match with Ryou, Kumi is seen to be very scared and nervous as she watches her brother take a beating. But despite Ippo's budding crush towards Kumi, he fights and defeats him relentlessly.
After the Rookie Tournament, she started working in a hospital in which she saw Ippo after the fight with Volg. This was her first appearance in a while, and Ippo was quite surprised to see her not only being kind to him, but even going on a group date with three of her fellow nurses and Ippo's three gymmates. It is here that Ippo is invited to her home after dropping her, and speaks briefly to her, before being suddenly interrupted by Mashiba himself. Ippo leaves soon after upon meeting him (fortunately encountering no physical hostility) and in the coming weeks gets closer to Kumi. She calls Ippo over to celebrate her brother's championship victory, and begins to even go on dates with him. In Ippo's fight for the Japanese belt, she is seen cheering him on. She is also seen at the celebration.
Before Ippo's first defense, he is pressured by his gymmates to confess, and ultimately almost does so as he is alone with Kumi in the snow. However he chickens out, leaving her visibly upset as she was expecting it. She is also angry that he seems to not care who she cheers for, and only makes up with him after his victory over Sanada. Like this, the two steadily grow closer over the years, until their romance is obvious to all who see them, even though neither are officially together or have really been together as a couple. Some notable moments of their closeness are when Ippo sits with her as she watches her brother's match in which he gets suspended, whenever she tends to his injuries after a fight, and when the two almost kissed but were rudely interrupted as the sled Kumi was lying on fell down.
This resulted in Kumi snapping mentally, as she realized a chance like that was just too rare. Comically, she began snapping at any mention of the word 'fall' and seemed to have a break down. One note about her relationship with Ippo is that over the years she occasionally wonders if he even has feelings for her, as he seems too focused on boxing, has never made any kind of romantic move to get closer to her, and seems too timid and too slow in general. However she excuses this, as the slow pace in a way suits her and because she finds his behavior endearing in its own way.
In New Challenger, she accompanied the Kamogawa gym to the beach. It was then that Ippo confessed his feelings to her, but she didn't hear due to her falling asleep.
She is beautiful with short brown hair and dark colored eyes, furthermore she isn't tall like her brother. In New Challenger, her body gains a paler/lighter skin tone.
In contrast to her brother, Mashiba Ryou, Kumi is quite cheerful and pretty. She first appears as a high school student and attracts the attention of many boys. However, her overprotective brother scares most of them off, resulting in her not having a boyfriend throughout her school years. She does not like boxing, but seems to have accepted it since the Ippo/Mashiba fight, which is also when she met Makunouchi Ippo. She disappeared for awhile afterwards but reappeared as a nurse after Ippo's fight with Volg Zangief. As a nurse, she often worries about Ippo and the other's wounds and even occasionally examines Ippo after his fights.
After meeting Ippo her feelings towards boxing have calmed considerablly, but she still retains her basic feeling of wanting him and her brother to retire. She is often unable to cheer for even her brother and seems envious of Itagaki Nanako, who can cheer for her own brother Manabu quite easily. Also, after the Mashiba/Sawamura bout she told Ippo that she doesn't believe she will ever like boxing.
She and Ippo are in love with each other, but Ippo's clumsy personality and thickheadedness prevent their relationship from progressing at all, as well as the numerous intrusions from Nanako and Iimura Mari. Particularly, Nanako intentionally distrupts their time together, often claiming to be looking for Itagaki or just suddenly noticing them while walking by.Although mostly different from her brother, when he went out to go clothes shopping with someone (Itagaki), she immediately called Ippo up to help her follow him, showing that she actually worries about him the same way he does for her. As she has become an adult she has also become a stronger woman, and has developed a habit of throwing objects at people (usually Mashiba) when her temper raises. She's arguably the only character in the entire series who has no reason to be afraid by her brother, and her comfort around him reflects this.
- "These are hot. They're fresh out the oven!" - Round 16, to Ippo during their second conversation at the Yamanaka Bakery