Mitsuri was born among five siblings and displayed an unnatural strength at a very young age through her unique muscle composition, shocking even her mother who herself was known for her strength. She grew up eating much more than the average person and developed an unnatural hair color (stated to be a result of eating too much sakura mochi), which led to her being seen as a freak and unable to proceed with a previously intended marriage when she was seventeen. After this rejection, she dyed her hair black, pretended to be weak, and forced herself to eat a normal amount in an attempt to fit in more with other girls, even if it negatively impeded her well-being. Shortly after making these adjustments to her physical appearance, Mitsuri was approached by a potential partner. However, she became doubtful over whether or not pretending to be a different person would truly work out for her in the end.
Mitsuri then encountered Kagaya Ubuyashiki, who told her to reject normalcy and embrace who she was as her strength was a gift. Because of this encounter, she joined the Demon Slayers and used her skill and ridiculous strength to rise to the status of Hashira. While in the Corps, she also adopted her idea of finding a man stronger than her to marry, however, this did not work out because, with the exception of her fellow Hashira, she was much stronger and more skilled than the rest of her fellow members.
At the Ubuyashiki Estate, Shinobu Kocho notes that Tanjiro Kamado is on trial, while Mitsuri thinks about the potential killing of Nezuko Kamado with uneasiness. When Obanai Iguro demands to know what punishment Giyu Tomioka would receive for his role in their subordinate's actions, Mitsuri praises his persistence, as well as Giyu's aloofness. Later, she questions whether or not they should make a decision in Tanjiro's trial without "Oyakata-sama's" input, suggesting they wait until he arrives. Shortly thereafter, she praises Sanemi Shinazugawa's increase in scars and Shinobu's sternness with the former's actions. The Love Hashira's expressions of praise quickly turns to horror as Sanemi stabs Nezuko, but just as quickly turns to laughter when Tanjiro headbutts him, much to her mortification.
When Tanjiro Kamado reaches the hidden Swordsmith Village, he loudly thanks a Kakushi for safely taking him there. Mitsuri, while relaxing in a hot spring, states that she hears the echo of a thank you and her heart is pounding. Later, after Tanjiro speaks to the chief and starts making his way up to springs, the Hashira sees Tanjiro and come running towards him in tears. She tells Tanjiro that she tried to ask for the name of a boy, but he 'ignored' her even despite her status as a Hashira. After complaining that the high spirits she received from the hot spring was lost, Mitsuri is told by Tanjiro that their dinner is almost ready and that it might include matsutake mushrooms. Mitsuri is immediately cheered up and walks away happily singing a song.
Tanjiro, Mitsuri, and Nezuko eat dinner together, and Tanjiro is amazed by how much the Hashira can eat even despite her claims that she didn't eaten as much as usual. Her appetite encourages Tanjiro to also eat a lot and become stronger. Tanjiro then remembers his encounter at the springs and tells Mitsuri that the boy she met before was named Genya Shinazugawa, and Mitsuri immediately realizes that Genya is actually the younger brother of the Wind Hashira, Sanemi Shinazugawa, even though Sanemi had told her he did not have any siblings. She figures that the brothers do not get along, and that it must be a sad experience since she gets along well with her five siblings. Tanjiro becomes worried for Genya since he still had not arrived to eat his dinner. While playing with and hugging Nezuko, Mitsuri assumes that perhaps the reason why he wasn't present was that he told the Swordsmith Village's townspeople that he will not eat at all. Tanjiro decides to bring Genya leftovers, and Mitsuri enthusiastically supports the idea.
Mitsuri sings while holding Nezuko's hand, and Tanjiro suddenly asks her what her motivation was behind joining the Corps. At first, she is embarrassed and hesitant to tell him but soon after confesses that she did it to find a man who would marry her for the rest of her life. She goes on to blissfully speak about how girls like men who are stronger than them and can protect them, and joining the Hashira would allow her to meet many strong individuals. Tanjiro is surprised and speechless, noted to have the same reaction as every other person who heard her reasoning had.
The three continue to look for Genya when a Kakushi comes up to them and tells Mitsuri that she is needed at the swordsmith workshop for the finishing touches on her sword. Saddened at having to leave the Kamado siblings, Mitsuri denies Tanjiro’s offer to escort her out and replies that she will be alright. She gives him a kind smile and tells him that even though they might not see each other alive the next time they meet, they should continue trying their best. She praises Tanjiro on surviving a battle against an Upper Rank, an invaluable experience that would translate to an incredible boost to his training. Mitsuri also states that he is much stronger than when they last saw each other and that she completely supports him and Nezuko. Tanjiro thanks her and announces that he couldn't have won without Tengen Uzui's help, but he will continue working harder to defeat Muzan. Mitsuri is happy to hear his words and whispers in Tanjiro's ear that there is a secret weapon in the village that can make him stronger, advising him to look for it. She then leaves with the Kakushi and bids farewell.
When the village is attacked by Upper Rank 4 and 5, Mitsuri quickly returns to the Swordsmith Village from her own district while Tanjiro, Nezuko, and Genya are in the process of fighting Hantengu's different forms and Muichiro battles Gyokko. She races towards the main houses where villagers are being attacked by Gyokko's mutant Demons and swiftly cuts them down. She finds a humongous, mutant fish-demon beginning to crush the chief of the village after already killing his demon slaying guards. A bloody bystander tries to get up to save him, but Mitsuri tells him not to get up because his internal organs are most likely wounded. She uses Love Breathing: First Form - Shivers of First Love to easily kill the demon and says that her heart "will never throb for those who hurt people with sick jokes." Tecchin falls from the mutant-fish's grasp, Kanroji safely catching him. Her eyes tear up as she tries to see if he is still alive, and the chief comically says that he must be in heaven since he is being held by a cute girl. She seems surprised and calls him a little perv as another wounded villager asks her to take his hand since he fell on his head too, Mitsuri instantly asking if he is okay. The bystander is immediately relieved.
When Tanjiro is swallowed by one of Zohakuten's tree dragons, Mitsuri slices the creature's head in half, carrying Tanjiro on her back safely. She squeals at the monster's size and questions what it is, then asks Tanjiro if he is alright, and that it's a good thing that she made it in time. After putting Tanjiro down, she tells him to rest and doesn't hear him try to tell her that the demon is the Upper Rank 4. She confronts Zohakuten, telling him in a disciplinary way that his "pranks" have went too far, and she will be taking Nezuko and Genya. Zohakuten tells her to shut her mouth, that he only takes orders from one person, and calls her a harlot. Kanroji is visibly in pure shock from his rude comment, thinking that he looks as young as her little brother, even though most demons are much older than they seem.
The hatred, emotion clone lashes at Mitsuri with Crazed Cry of Thunder Death. Tanjiro calls out to her and, without hesitating, she uses Love Breathing: Third Form - Catlove Shower against it. She declares that she is getting angry, and, though Zohakuten may look like a child, his actions are still unforgivable. Tanjiro is shocked at how well she suppressed his bolt, and the demon seems a bit impressed by how she cut through his attack itself.
Mitsuri continues to battle against Zohakuten. She dodges the crushing hit of the maple leaf-like fan and uses Love Breathing: Second Form - Love Pangs. One of the tree dragons throws another attack from its mouth, but she evades it with Sixth Form - Cat-Legged Winds of Love. The demon realizes that she is too fast and is keeping up with his attacks, making him have to use a technique. He sends Blood Demon Art: Countless Striking Trees, lashing out many wooden dragons. Mitsuri becomes a bit nervous at how wide-ranged it is and questions if she can get past it. She strikes back with Love Breathing: Fifth Form - Swaying Love - Wildclaw. She gets past the attack and comes very close to Zohakuten. The demon opens his mouth, confusing Kanroji, though she brushes it off and goes for his neck. Tanjiro yells to her that he is not the real body, and he will not die if his head is cut off. The Love Hashira realizes her mistake and is fiercely hit by Zohakuten's Compressed Sound Waves. She loses consciousness and slumps before the Upper Rank. Tanjiro screams out to her as Genya and Nezuko try to break free from the wood and help her. The demon is flabbergasted by how Kanroji maintained her body shape and withstood his head-on attack. He notices that she might've tensed up all her muscles before his attack, and that she is no ordinary girl, her strength not matching her looks. He thinks she has "high-quality meat" but decides to crush her brains and skull in, pulling back his fist. As he is about to kill her, her entire life flashes before her eyes.
She wakes up as Tanjiro, Nezuko, and Genya fall on her and pull her away from Zohakuten. Tanjiro exclaims that they have to protect Mitsuri, because she is their ray of hope and only chance of winning, and that none of them are going to die. The demon launches another quick attack, but Mitsuri cuts through it all. She cries as she thanks everyone and says sorry for messing up, even though she is a Hashira. She says she won't let any of her friends die, the demon slayers are her precious home, she will not lose to "evil people", and that it's time to get serious.
Mitsuri remembers everyone who has been a supporter in her life and states she will protect everyone. She runs at the demon, slicing through his wooden dragons. She thinks to her herself that she needs to get her blood circulating and increase her heart rate to become faster and stronger. Zohakuten tries to hit the siblings and Genya, who are trying to find the main body, but the Love Hashira stops him. The Upper Rank notices how much faster she has gotten and wonders how she did it. Then, he sees some sort of birthmark close to her clavicle that he hasn't seen before, resembling a demon crest. He throws another attack, but Kanroji continues to cut it, making him extremely annoyed that he can't kill the others. He knows that she won't last forever and that, sooner or later, she will run out of stamina.
Right as Kanroji feels as though she is going to die, Tanjiro kills the main body, and Zohakuten crumbles to dust. Mitsuri falls to the ground and cries tears of joy, exclaiming that they must have cut Hantengu's head off. She runs up to Tanjiro, Nezuko, Muichiro, Genya, and Kotetsu and pulls them into a group hug. She cries and rejoices that they won, and that it is awesome. She wails that they're alive and thanks the gods with Nezuko repeating after her.
There is an emergency Hashira meeting held in the Ubuyashiki Estate. Obanai asks how both Mitsuri and Tokitou are doing. Kanroji declares that she is feeling a lot better and happily thinks to herself that they're worried about her, while Muichiro says he is not at his full strength again yet. As the Hashira discuss amongst themselves, Amane, Kiriya, and Kanata Ubuyashiki arrive and begin the meeting in place of the ill Kagaya Ubuyashiki, Amane apologizing for the wait. Mitsuri, along with the rest of the Hashira, bow before Amane.
Amane speaks about how Mitsuri and Muichiro had gotten Demon Slayer Marks during their battles, and she asks if they could explain how to get them and what the conditions are to obtain them. Mitsuri questions what she means by marks, and Amane explains to the Hashira. When asked again to inform them how to get a mark, the Love Hashira thinks to herself that Mrs. Ubuyashiki is wonderful, and she starts to explain very excitedly how she felt during her fight. She enthusiastically states that her body was "super light", and that it was like, "badaaaaam", "duuurrr", "my ears went screeeeee", and more exaggerated sounds. There is an awkward silence, as everyone is very surprised by her explanation. Very embarrassed, Mitsuri bows her head to the ground and says she is extremely sorry and that she will go hide in a tiny hole.
When Muichiro tells of his state, Mitsuri turns her head to him and looks at him in surprise. As he explains deeply, she thinks to herself that she sees. Amane thanks them, and she says she has one thing to say regarding their mark training. Mitsuri tilts her head slightly and asks what it would be, with Amane answering that those who have displayed the mark must participate. After the Ubuyashikis leave, Tomioka says he that he should leave as well. Mitsuri holds out her hand toward Giyu and seems reluctant for him to leave. When Sanemi is angered by the Water Hashira's words, Kanroji says "no fighting" nervously, but Shinazugawa charges toward Tomioka. Mitsuri exclaims no multiple times, before Gyomei Himejima loudly claps his hands and stops everyone in the room.
Tanjiro narrates later on that, after Tengen's stamina training, Kanroji handles the flexibility training. He also says that those who already have the mark will train so they can continuously stay in their 'marked state', as a picture of Mitsuri standing on Muichiro's and Tanjiro's knees and Nezuko laying on the ground is shown.
When Tanjiro reaches the Kanroji Residence, Mitsuri waves to Tanjiro and happily welcomes him into her home. He thanks her and says he is glad she is doing okay; she replies that she feels the same way. Tanjiro asks if she keeps bees at her home and claims that he smells honey. She proclaims that he figured it out, expressing her love for making honey and putting it onto her bread. She leads him to the kitchen while she says she loves her bread with tons of butter too and to pour black tea in the afternoon. Mitsuri tells Tanjiro that she will make him pancakes, but he is confused on what the foods she is saying are. During her training, it is shown that Mitsuri makes all members dress in ballet clothing and elegantly dance nonstop to the beat of music. Tanjiro describes flexibility as "hell", where she apparently stretches everyone with just her strength.
As Kagaya speaks to Muzan Kibutsuji about the sleeping creature that absolutely despises him, a picture of Mitsuri, Tanjiro, Nezuko, and all the other Hashira is shown. When there is an emergency summon, Mitsuri, along with her fellow demon slayers, rush toward "Oyakata-sama", in the hopes that they may reach him in time.