Kanao lived an impoverished lifestyle before she was taken in by Kanae Kocho and Shinobu Kocho. Her parents were incredibly abusive towards her and her siblings, beating them and drowning them if they did anything they disliked. She broke psychologically, becoming an empty shell who was sold off into slavery.
As she was being led off, tied around the waist by a rope held by her purchaser, they were approached by the Kocho sisters. After confirming she was a slave, Shinobu distracted the man by flinging money in the air. When he released the rope in surprise, she grabbed it and the sisters escape with Kanao to the Butterfly Mansion. Soon after, the sisters discover the extent of her trauma, which makes Shinobu view her as useless, if not potentially dangerous. Kanae, unperturbed, gives Kanao a coin and instructed her to flip it if she ever needs help making a decision.
She entered the Demons Hunters, since she wasn't able to do any housework or nursing at the Butterfly Mansion like Aoi and the others could. Kanae and Shinobu didn't condone it, but Kanao learned the Breath of Flowers just by watching them, and she took the selection exams without their permission.
After Kanae was killed by Doma, Kanao was at her grave with the rest of the members of the Butterfly Mansion. Although she was disturbed by her own inability to cry at her beloved sister's death, nobody judged her for it.
Kanao appears as one of the nearly twenty participants in the Final Selection, and is one of the five to survive. She says nothing in her appearances, merely gazing at a butterfly and ignoring the commotion around her.
As Shinobu's Tsuguko, Kanao accompanies her and Giyu Tomioka to Natagumo Mountain on Kagaya Ubuyashiki's orders. She pursues Tanjiro and Nezuko Kamado when they attempt to flee from Shinobu, successfully catching up and knocking an injured Tanjiro unconscious after he tells Nezuko to run. She catches up to Nezuko as well and tries to cut her head off, but Nezuko dodges by shrinking into child-size and continuing to run. The chase continues as Kanao fails to land her subsequent hits, and through all of it, Nezuko doesn't attempt to fight back. Though Kanao is left baffled at the idea of a Demon not attacking humans, she decides not to ponder on it since her orders were to kill any and all Demons. Once she receives orders by a kasugaigarasu to capture the siblings, she immediately stops attacking and asks the Demon if she is Nezuko.
As Tanjiro and Nezuko arrive at the Butterfly Mansion, Kanao is seen standing quietly in one of the gardens, surrounded by butterflies. When the Kamados' escorts asks her if they can enter the Mansion, she simply stands there silently, a vacant smile on her face, until Aoi Kanzaki comes over to let them in.
During Tanjiro, Zenitsu and Inosuke Hashibira's daily recovery training at the Mansion, Kanao takes turns with Aoi to play the cup game with each of them, a game of speed in which the aim is to throw sour medicine over your opponent before they do you, as well as a simple game of tag, and she beats her opponents every time in both games. Her winning streak continues until Tanjiro begins training his total concentration breath techniques to increase his speed and stamina, after which he is able to win both games against her. In later training, it's stated that the two had a fairly even streak of wins and losses to each other as they continued to play.
After the trio had make a complete recovery and begin to prepare to leave the Butterfly Mansion, Tanjiro finds Kanao sitting alone in one of the gardens, and walks over to her to thank her for all her help with their training. Kanao smiles up at him, but says nothing, even as he goes on to praise her for her strength and status as a Tsukugo. After more silence, Kanao reaches into her pocket and takes out her coin, flipping it high into the air. It lands on tails, and, after seeing the result, she looks back up at Tanjiro and says that he needn't thank her since she just followed her instructor's orders, and waves him goodbye. An amazed Tanjiro asks what the coin was and why she flipped it, to which she replies that it's something she uses to make decisions when nobody's ordering her. Confused, Tanjiro askeds why she needed to do this, wondering what happened to make her like this, and Kanao just dismisses him, saying that it doesn't matter. After some thought, Tanjiro asks her if he can borrow her coin, and, after receiving it, flips it high into the air and says that the result will determine whether or not she'll listen to her own heart from then on, instead of just following instructions. He calls that it will land on heads, which it does, and Tanjiro cheers, grabbing Kanao's hands and telling her that she's now free to live how she desires. Before Kanao can reply to this, Tanjiro waves her goodbye and runs off over the garden. Kanao calls after him, asking why it had landed on heads since she didn't notice him using any tricks, and Tanjiro turns, smiles at her, and says that it was totally by chance, before continuing his exit.
Tanjiro finally awakens from the coma he was in due to his injuries at Gyutaro and Daki's hands, and is met by a shocked Kanao, who drops the vase she was holding and rushes to his side, asking him if he's okay. Kanao explains to him that he was unconscious for a whole two months and smiles as she says that she's so glad that he's finally awake. Goto enters the room with the castella he brought over, and Tanjiro thanks him, which greatly shocks the kakushi, who then yells to Kanao that she should've told someone Tanjiro was finally awake, before also yelling the news to Kiyo Terauchi, Naho Takada and Sumi Nakahara. The three come rushing over, cheering and crying, glad that Tanjiro is finally awake, and Aoi joins them shortly after, expressing her guilt and gratitude to Tanjiro for going to the Red Light District in her place.
Tanjiro asks them how everyone else is doing, to which Sumi and Goto reply that Zenitsu had awoken the day after the incident and had gone back on the job two days ago, and Tengen had made it back without any help from the kakushi, just with the support of his wives. Tanjiro expresses his gladness that they're okay, and asks how Inosuke is doing. Sumi and Aoi explain that Inosuke was very close to death at one point due to his bleeding and the poison circulating around his body, and Tanjiro says that he must just be hallucinating the boar-headed Demon Slayer on the ceiling of the room then. Everyone looks up, realising that Inosuke is indeed there, as he jumps down and lands at the end of Tanjiro's bed, saying he's impressed Tanjiro noticed him and boasting that he woke up a whole seven days before he did. As Inosuke continues ranting at him, Kiyo shows Tanjiro a picture of a honey badger and says that Shinobu said Inosuke was just like it, since he's nearly impossible to kill. Inosuke exclaims that he's immortal, but Goto says he's just a moron, which stirs up commotion in the room, with the former trying to attack the latter as Aoi attempts to hold him back. As they continue to argue, Kanao tries to tell them to be quiet, but they ignore her until she raises her voice and shouts that Tanjiro has fallen back asleep so they should let him sleep peacefully.
During the raid on Muzan's fortress, Kanao fights Doma in the wake of Shinobu's death, having previously been told of Shinobu's plan to defeat Doma; the plan required that Shinobu be killed and consumed by Doma in order to deliver a lethal dose of wisteria poison to the Upper Moon in order to weaken him.
Shinobu is absorbed by the Upper Moon just as Kanao arrives at the scene, but just before her death she manages to signal to her Tsuguko with her hand. Her death causes Kanao to go berserk in order to try and save her, only to be repelled by an irritated Doma, who tells her not to interfere when he's in the middle of absorbing someone. He successfully devours Shinobu's body until her butterfly-shaped hair piece is the only thing left of her.
Doma notes aloud that Kanao must have been told something by the signal Shinobu gave her shortly before her death, wondering what she could have told her, as Kanao stands silently before him, enraged. As he licks Shinobu's hairpin, he says that it is a good night, as such fine feasts keep showing up one after another.
After Akaza's death, Doma says that he felt as if the Upper Moon had turned into a different creature before he died, and that his demise was inevitable because he never ate any women. Doma begins to cry over his 'best friend's' death, but Kanao just tells him to stop with the lies. She explains that she knows that every word that comes out of his mouth is nothing but random babble, and that it's impossible for him to be genuinely sad over Akaza because he can't feel anything at all. She says that every person born into the world has feelings, but Doma cannot understand these feelings and covers this up by spitting lies. He has never felt happiness, joy, pain and bitterness, and she says that he is nothing but an empty shell, and asks Doma why he was even born. Doma's face turns serious, and asks how Kanao can say something so cruel, to which the young Demon Slayer responds that she simply hates the Demon and wants to send him to hell this instant.
Suddenly, Doma appears behind Kanao, attempting to use one of his fans to cut her head, but she quickly dodges and delivers a blow to his stomach, causing some of his organs to fall out. He doesn't seem to feel her blow, simply sending some of his ice lotuses at her and noting that her reaction speed has risen as she's able to adapt to his attacks. As he regenerates his body, he thinks to himself that Kanao could actually be stronger than Shinobu, a Hashira.
Trying to suppress her rage, Kanao attacks Doma with Breath of Flower: Peony of Futility, which he avoids with ease, smiling and commenting on how pretty her attack is before sending his Blood Demon Art: Wilter Hanging Garden at her, which she barely fends off with Breath of Flower: Plum Spirit. Doma notices that she is only able to counter and dodge his attacks because of her sharp eyes, since she takes note of the subtle movements of his shoulders, gaze, toes, elbows and knees to predict what he'll do next, and decides it's best to prevent this by taking her eyes out with a swing of his fans. Kanao dodges, and charges at him with Breath of Flower: Swirling Peach which is nullified by Doma's next Blood Demon Art: Freezing Fog. The Upper Moon sends two more attacks, Lotus Vines and Arctic White Princesses, her way, which she only just avoids before his Wintry Icicles begin to rain on her from above.
Kanao dodges again, moving farther away from the Demon, who laughs at her and says that she can't keep moving backwards if she wants to cut off his head. At a speed too quick for the eye to process, Doma runs at Kanao and snatches her blade, returning and planting it in the ground a distance away from her. He says she better come and get it quickly if she wants to survive him, before sending his Scattering Lotuses at the Demon Slayer.
At that moment, Inosuke breaks through the ceiling of the room, protecting Kanao from Doma's attack with his Breath of the Beast: Mad Cleave. He looks at Doma's eyes and notices that he's Upper Moon Two, which he screams out, happily shouting that if he beats the Demon he'll become a Hashira and asking Kanao what his Hashira title should be. Inosuke notices Kanao's beaten-up face, and asks, surprised, what had happened, saying that Shinobu will get mad if she finds her Tsuguko in that state. Kanao remains silent, and the boar-headed Demon Slayer comes to a realization and asks her if Shinobu is dead, to which Doma replies that she isn't; she will live inside of him for all eternity because he has "saved" her from her pain and suffering. Inosuke recalls Shinobu's kindness to him, and vows to Doma that he'll kill him as he rushes towards the Upper Moon with his Breath of the Beast: Mince to Bits.
Kanao shouts out a warning to him not to breathe in Doma's cold air as the Demon remarks on how messed-up his technique is. Doma attempts to slice off Inosuke's head with one of his fans, but the Demon Slayer flips away from him, launching himself from the flat edge of the fan and landing next to Kanao. He hands her her blade, which he had taken back from Doma, and tells her never to let go of it again. Doma smiles and compliments Inosuke's speed as he jumps towards him, and the two swiftly engage in quick combat, launching Breath Styles and Blood Demon Arts at each other at alarming rates. Inosuke kicks Doma in the stomach, sending him skidding backwards as he dislocates the joints in his right arm and prepares to swing his sword at the Upper Moon's neck. He misjudges his swing and instead slices through Doma's eyes with his Breath of the Beast: Godly Undulating Slice, and the Demon laughs at his unusual technique as his joints relocate.
Doma calls Inosuke messed-up and says that he's never met a kid like him before despite his long lifespan, to which the Demon Slayer replies that that's obvious, since he is the great Inosuke. At an incredible speed, Doma lurches forward and snatches his mask from his head as he's speaking, and Inosuke, enraged, yells at him to give it back. The Demon notices his face, and says he remembers Inosuke from somewhere, asking him if they'd met before. Inosuke responds that they hadn't as he would remember meeting someone like Doma, and Kanao agrees that he was likely making it up due to his tendency to fake things. However, the Upper Moon insists that they have, proceeding to stick his finger through his skull and into his brain to try and jog his memories, which Kanao and Inosuke are visibly disgusted by.
Doma says that he knew Inosuke's mother, as his cult took her in, but Inosuke denies that he has a mother since he was raised by boars. Doma laughs and says that he must have a human mother since he's a human, and Inosuke shouts at him to quit spouting lies and give him back his boar mask, rushing in to attack the Demon again with Breath of the Beast: Jagged Gnaw. His torso is slashed twice by Doma's fans, who smiles slyly and tells him not to interrupt before sending him flying back to Kanao with his ice.
Doma then tells the story of Kotoha, Inosuke's mother, and how she and her baby son joined Doma's cult only to end up running away after she realized the Demon was eating his followers. He explains that she met her demise by Doma's hands, who still believes he was saving her from her abusive household by eating her, but dropped her son off a cliff before the Demon's attack in the hope that he would survive. Memories of her begin to come back to Inosuke, who recalls the song she used to sing to him. Doma calls Kotoha stupid and unfortunate, saying that there wasn't a happy moment in her life, and Inosuke, now enraged, thanks Doma for allowing him to remember this story and promises to not only to kill him for what he did to Kotoha and Shinobu, but to put him through hell beforehand.
Doma smiles again, and tells Inosuke that there is no heaven or hell in this world since they're nothing but fantasies created as a coping mechanism for weak humans, as Kanao stares at him in silence, enraged. Inosuke shouts that he doesn't care if there's no hell because he can just make one himself, and tells Doma not to talk about his mother like she was unhappy.The Demon sends his Blood Demon Art: Freezing Fog at Inosuke, which he blows away with Breath of the Beast: Circular Fangs as Kanao prepares to use another Breath Style to attack him from behind. Doma bats her away with one of his fans and moves away, and Inosuke yells at him to get back down so they can continue fighting. He replies that he can't, since Akaza has been defeated and he doesn't have time to play anymore, and uses his Blood Demon Art: Crystal Prince to create smaller clones of himself for Inosuke and Kanao to fight instead. The clones attack the Demon Slayer duo with Doma's techniques as the real Doma starts to walk away, the clones using his Scattering Lotuses, Lotus Ice and Arctic White Princesses attacks to hold the duo back. Now at the exit, Doma thinks about how Inosuke has managed to avoid breathing in the icy air from his techniques, and takes note of his sharp senses, saying he will make sure to remember him. He spectates their battle for a few seconds, observing how they use each technique so that he'll be able to kill them quickly the next time they fight, and then turns back towards the door. Doma decides to create three more clones to let loose around the fortress in order to kill the rest of the Demon Slayers there, and overhears Kanao shouting to Inosuke that they have to hold their ground just a little longer against the crystal princes. He wonders what this could mean, but then dismisses it and begins to reach for the door handle.
Suddenly, Doma's face begins to melt. He drops to the floor, one of his arms snapping off, and his crystal princes begin to crack and disintegrate as he wonders what's happening. Inosuke asks if it's a trap but Kanao, having been informed prior to the battle of Shinobu's plan to kill Doma, tells him that it isn't, and that the Demon is being melted by the poison Shinobu had in her body when he consumed her. She tells Inosuke to aim for his neck to finish him off, and, in a panic, Doma plays his final and most powerful card, Blood Demon Art: Hoarfrost — Waterlily Bodhisattva.
Doma's ice Bodhisattva statue slams down its hand on the floor of the room, creating a small earthquake that sends Inosuke and Kanao up into the air. Kanao notes that his technique lacks precision, and was clearly the Demon's last effort to protect himself with the little energy he had left, deciding to use her final form, Breath of Flower: Equinoctial Vermilion Eye to try and break through him. Doma's statue grabs Inosuke in one of its hands as Kanao leaps towards the Upper Moon, hitting her blade against his neck. She is unable to cut it, so Inosuke, on the spot, comes up with and uses Breath of the Beast, Improvised Fang: Throwing Slices to hurl his blades at Kanao's to give it the extra strength needed to cut Doma's neck. They succeed in beheading him, and Upper Moon Three and his statue begin to disintegrate.
After successfully defeating Doma, Kanao searches the ground for Shinobu's lost hairpin, and realizes that Kanae's has broken. She recalls that when Kanae died, she was unable to cry, but no one judged her for it. Kanao remembers how Kanae had advised her to value her comrades, and wonders if she did so correctly this time, as she cries for Shinobu. Once she finds Shinobu's hairpin, Kanae and Shinobu's spirits assure her that she has done well.