When Kyoko was a child, she was used as a bait to lure out his brother, Ryohei Sasagawa, by upperclassmen who disliked him. As a result, Ryohei was injured when he tried to protect her and the injury left a scar on his head, making Kyoko feel guilty. Not wanting to see Ryohei hurt again, she asked him not to get into fights again, something that Ryohei promised but is unable to keep to this day due to his position.
When she was a child, she dreamed of becoming a prima ballerina and dancing in Swan Lake in France, though she also stated she'd like to become a police officer as a second option.
Kyoko's family, besides Ryohei, is somewhat unexplained, with only subtle mentions to her parents. Kyoko had also admitted to being somewhat distant to her cousins when she first believed that Reborn was Tsuna's cousin.
In Namimori Junior High, Kyoko is considered to be the most popular student and is greatly admired by Tsuna, who considers her the only reason to attend school. This is despite Kyoko being hardly aware of Tsuna's existence, though she was the first girl to have ever talked to Tsuna. She first takes interest in Reborn, who she deems to be cute, and doesn't show any concern over Reborn bluntly explaining his status as a hitman. Kyoko carries on to school, where she is quickly stopped by Tsuna in Dying Will Mode, who asks Kyoko to become his girlfriend. This fails as she runs away out of fear due to Tsuna's loss of clothes and wounded face.
Kyoko later attends the kendo match against Tsuna and Kensuke Mochida the next day, by force from Hana Kurokawa and another unnamed friend, who tease her for "crying" when coming across Dying Will Tsuna, and being in love with Kensuke, despite her rebuttal. Kyoko doesn't take the kendo match very well at first, as she is named a "prize" by Kensuke for whoever wins the kendo match, and she attempts to lunge for him, only for her friends to restrain her. When Kensuke is defeated by Tsuna in the kendo match, Kyoko expresses her admiration towards Tsuna, and apologizes to him for previously running off, as she believes that Tsuna's love confession was merely a joke, while Kyoko remains unaware of Tsuna's feelings for her, the two become friends.
Sometime later, Kyoko visits a coffee shop, where she comes across Reborn once again, and offers to pay for his espresso, oblivious that Reborn had attempted to shoot an employee. Kyoko visits Reborn at [Tsuna's house, where she is paid back by Reborn, she assumes that Tsuna and Reborn are brothers, and later cousins, and believes that Tsuna's mouth foam is fake when Reborn strangles him for being touched. They are interrupted by Nana Sawada, who is surprised that Tsuna had a girl in his room, and seemed grateful that her "no-good son" finally found a girlfriend, which Kyoko quickly denies, though seems to be angered by Nana for calling her own son a loser. Tsuna assures Kyoko to disregard his mother, and Reborn changes the subject, suggesting that Tsuna and Kyoko play Russian roulette, which Kyoko agrees to as she believes that it sounds fun after Reborn reveals that the gun contains a "party bullet" and is completely safe. Kyoko shoots herself in the forehead, revealing the "party bullet" to be a lie, as the gun was actually loaded with a Dying Will Bullet. Kyoko is brought back to life in Dying Will Mode, and she reveals her regret for allowing Nana to belittle Tsuna. She forces Nana to take back her "no-good son" comment to the point of her foaming at the mouth and fainting. Reborn takes Kyoko out of Dying Will Mode with his Reverse One Ton Mallet, which he uses to knock the Dying Will Bullet out of Kyoko's head. Sometime later, Kyoko assumes that the previous events were merely a dream. While in a bakery, Kyoko encounters Tsuna much to her embarrassment, she explains that it's an appreciation day where she takes the third Sunday of a month to eat as much cake as she likes.
Coincidentally, Haru Miura, who was with Tsuna in the bakery, also has a similar appreciation day. Kyoko instantly becomes friends with Haru, and the two eat their cakes at Tsuna's house. The two take an interest in I-Pin's Gyoza Buns, but both fall unconscious due to the interference of Bianchi's Poison Cooking. To prevent the two from dying, Reborn shoots them both with a Dying Will Bullet, and the two are both reborn due to their regret of only eating one piece of cake on their appreciation day. Reborn performs the Reverse One Ton Mallet technique on Kyoko and Haru, and the two believe the previous events to be a dream when they wake up. Later that night, Kyoko is shocked to find out that she put on weight and cancelled any future appreciation days.
Kyoko visits her Ryohei in the hospital after he was attacked by Ken Joshima, where she at first believes the story that Ryohei was injured after climbing the chimney to the bathhouse and received a "sprain" after falling, and also believes at first that his gauze, broken arm, and missing teeth are also part of the "sprain". Kyoko argues with Ryohei over the how it's impossible for Ryohei to be hospitalised over a sprain, no longer believing his lies. Kyoko begins to cry that she's happy that her brother is alive, but Ryohei quickly stops her.
When Tsuna and his friends encountered Birds in Kokuyo Land, the latter shows them a video of Haru, and Kyoko and Hana were stalked by his subordinates the Bloody Twins, and threatened to order the twins to kill them if Tsuna don't listen and do as he says. However, she and Hana were saved by Doctor Shamal while Haru was saved by Future Lambo and I-Pin whom Reborn had requested to watch over them.
At the end of Tsuna's flashback due to the Criticism Shot, he hears her being reassured by Ryohei that Tsuna will come back, so she agrees and states she had total faith in him.
Kyoko accepts Ryohei's story that he, along with the rest of Tsuna's Guardians (though he doesn't use that term), are just participating in a series of sumo-wrestling matches over the course of the next seven nights. She doesn't like it, but she accepts his reasons for coming home so badly beaten. She also takes care of Lambo with Haru and Nana after Lambo is badly beaten in the Lightning Ring Battle. Before the Sky Ring Battle, Kyoko gives Tsuna a handmade charm and wishes him good luck for the sumo-wrestling match.
In the future, the adult counterpart of Kyoko is escorted to safety by the teenage counterparts of Lambo and I-Pin away from Millefiore Famiglia mafioso, after an attack from Nosaru and Tazaru which makes her fall to a nearby warehouse. While mostly unharmed, adult Kyoko sprains her ankle, and is finds the present Tsuna to be nostalgic. Adult Kyoko refers to Tsuna as "Tsu-kun", though it's unknown why and she quickly reverts back to her younger counterpart before explaining.
In the present, sometime after the disappearance of Tsuna and Hayato Gokudera, Kyoko joins Takeshi, Haru, and I-Pin in search of their missing friends, and is sent ten years into the future. Present Kyoko is caught right in the middle of Tsuna's conflict between Tazaru, and is instantly used as a target, and saved by Tsuna in his Hyper Dying Will Mode. After Tazaru is defeated, and Kyoko is escorted back to the Vongola Base, Kyoko is informed about her situation of being stuck in the future by Reborn, who leaves out any details about the mafia or the Vongola Famiglia, since Kyoko doesn't have any fighting experience, she instead helps Haru in cooking and cleaning for Tsuna and his guardians while trapped in the future.
Later, concerned for her friends and family and feeling trapped inside of the base, she locates an unobserved exit and finds her way into the city, successfully avoiding capture by the Millefiore and taking shelter with Hana until Tsuna comes to find her. Kyoko and Haru volunteer to do the housework while Tsuna and his friends train. Kyoko and most of the other non-fighting characters stay behind in the invasion.
In the week leading up to the Choice game, Kyoko and Haru demand Tsuna that he tell them everything about the Mafia, Byakuran, and the Millefiore. While Bianchi, Futa, Giannini, I-Pin, Chrome Dokuro, and Reborn support them, Tsuna refuses to admit to them that there was anything worth knowing; as a result, Kyoko and Haru go on strike by refusing to do any housework while being kept in the dark.
She eventually recants, and on her way to buy groceries to cook food for everyone, she is stopped by Tsuna (who was misled by Bianchi to believe she had run away again). Tsuna realizes on the way to stop her that there is a difference from being kept completely in the dark and knowing there is something that others are refusing to tell a person; so, he sits her down and tells her everything. She is then able to offer her opinion as to why Tsuna's Sky Vongola Box was being temperamental, which gives Tsuna a clue on how he needs to open the Box Weapon correctly.
During the first Choice battle, she and everyone (except the participants) were watching the fights from the audience stalls.
When Ryohei was about to get into a fight with Kouyou Aoba, Kyoko happened to passing by Tsuna's house and heard the word "fist" that Ryohei said, questioning does he intend to fight, which Ryohei reluctantly answered that he is not. She then supported Ryohei when Nana suggested that they should have studying competition instead of fighting.
In an attempt to cure Kaoru Mizuno of his too shyness to play baseball in front of crowds, Reborn suggests to imagine everyone around him as vegetables, Kyoko joins in, dressed up as a carrot, which seems to work and give Kaoru confidence.
After the Inheritance Ceremony is cancelled, Tsuna comes across Kyoko and Haru at their usual cake shop, buying some cakes, seen by Tsuna. Reborn asked Tsuna if he wants to approach them before fighting against Simon Family, but Tsuna doesn't want to involve them again and leaves, quietly saying goodbye to them. Kyoko suddenly states that she feels warm, most likely barely sensing Tsuna's presence.
After one week has passed since the battle at Simon Island, Kyoko comes to school to see her Ryohei, in a net due to Adelheid's punishment for him breaking the rules; she was worried about Ryohei because of his injuries from a supposed "shooting star", afraid of him reopening his wounds, much to Tsuna's disbelief at the obvious lie Ryohei gave her.
Kyoko expresses her happiness when Chrome first transferred to Namimori Junior High, which is abruptly cut short whenever she begins crying in Kyoko's arms. Chrome explains that she was kicked out of the Kokuyo by Mukuro Rokudo, and was left to attend in Namimori, which Kyoko later explains to Ryohei. It's decided that either Kyoko or Haru's house until further notice. Kyoko later attended Chrome's welcoming party at Tsuna's house, in attempt to cheer her up after being kicked out of Kokuyo.
After the second day of the Representative Battle, Kyoko meets Tsuna and his representative members in the hospital, causing Tsuna to question her presence. Kyoko explains that Chrome wasn't at school that day, and that she found Chrome feeling unwell in her apartment and took her to the hospital for a checkup. Kyoko then reveals that the analysis stated that her internal organs were gone. Kyoko tells Tsuna that despite her grave condition, Chrome seemed to want to talk to him, always calling out for "boss."
Sometime later, Kyoko once again encounters Tsuna, who she finds sleeping outside, and asks Tsuna if she can borrow his good luck charm she made him in order to add more prayers, before departing with Chrome.
After Bermuda is defeated, and the Representative Battle had come to an end, Kyoko celebrated the occasion by creating a basket of plushies to pray for everyone's recovery. When visiting Dino and Basil in the hospital, Kyoko and the others decide to wait while Tsuna and Reborn visit all the injured representatives in the hospital. With everything now in peace once more, Reborn picks Kyoko as one of Tsuna's candidates as a girlfriend.
Tsuna is once again forced to confess his feelings for Kyoko in Dying Will Mode, but is interrupted by a construction site that is about to crash down onto Haru, Lambo, and I-Pin, which causes Tsuna to change his Dying Will and save Haru and the others. Kyoko is impressed by Tsuna, and remarks that he's not an ordinary person, the same thing she said when the two had first met. While Kyoko reaffirms her friendship towards Tsuna, she remains unaware of his romantic feelings towards her.
Kyoko's final appearance is with Haru, Lambo, I-Pin, Chrome, her brother, Takeshi, and Hayato standing outside Tsuna's house as they wait for Tsuna to join them for celebration of Tsuna's tutelage, invited by Reborn.