When a young Totsuko Haruta (肭子 — "new moon," "child,") lived in Ōsaka, she was married to Shiryu Kadokawa (紫龍 角川 — "purple, violet," "dragon, imperial," "horn, antlers," "river"). It wasn't a marriage Totsuko necessarily wanted or approved of. Shiryu was the fourth son of a wealthy family that seemed to have bad luck, as their other three sons had all perished while working in their family's factory. Despite this, the families wanted to merge their wealth, and Totsuko, being their youngest daughter and still unmarried, was the prime candidate. So they wed, and for some time, they were decently happy. Totsuko spent her time in the fashion and shopping districts, admiring the newly emerging Western influence. They had a boy named Kenta (憲大 — "law," "large"), and were planning on taking a vacation when once again, tragedy struck, and Shiryu was killed when working at the factory late one night. Devastated and haunted by the gruesome nature of his death, Totsuko decides she will go on with their scheduled plans, and takes her infant with her to Ikaruga in Nara prefecture.

The purpose of this vacation was to pray at the Buddhist temples of  Hōryū-ji and Hokki-ji for the safety and prosperity of Kenta. Instead, she prays for the sanctity and peace of Shiryu's soul, and that their families may know serenity from then on. Then a young man sits beside her and, moved by the intensity of her concentration, prays for the same thing. He introduces himself as Yukimura Higashihara (雪村 東原 — "snow village," "east plain"), and invites her to dinner at his family's estate, where he promises she and her son will be protected from any more misfortune.

Totsuko extends her time in Ikaruga to spend more time with the serious and severe man known as Yukimura. He is the first son of the Higashihara family, a demon slaying family known for producing talented slayers throughout the generations. Though he is solemn and rarely smiles, he does smile for Totsuko. The longer they spend together, the more she is sure that she was meant to marry this man, and so they wed. 

At first her family is scandalized, as her late husband has only just recently perished, and her in-laws are even more so, and then there is this business of demon slaying. Exactly what kind of future were Totsuko's children expected to have? Well, that one was easy. Yukimura already prepared Totsuko for the inevitability of their children's training. They must take on the demon slaying mantle.

Yukiko is born a few years later.

From the moment she learned to walk, Yukimura starts training her in the Breath of Silence. Kenta began his training earlier, but the sheer difference in their innate abilities is present almost immediately, and she catches up to him and surpasses him in no time. Yukiko is more flexible, more fluid. She takes orders more easily. The more praise she receives, the better she does, and this leads to resentment growing in Kenta, who spends most of his days being chastised by their father for his sloppiness and lack of skill. He takes it out on Yukiko by pushing her around or making fun of her, even when she is nothing but kind to him. Totsuko wishes to step in, but Yukimura stops her. They are children. It is a tale as old as time, brothers and sisters fight, but they will stand up for each other when the time comes. 

When Yukiko has manifested all six forms of her Breath Style, Yukimura sits her with him in the estate's great room and places in her hands the blade of their progenitor, Ishizu Higashihara (威静 — "power, authority, majesty," "quiet"). It is a beautiful, sparkling blade with the kanji for "silence" engraved in it, and though it is too big, she straps it to her side with so much pride that for the first time since she was born, Yukimura smiles at her. The only one who wasn't smiling was Kenta, who wanted to be the one to inherit the family's blade. He instead must carry a consolation prize, a blade made specially for him but wasn't special enough. Kenta delves further into his training, enough to injure himself, but not enough to overpower her. If only he could prove his worth somehow.

The opportunity presents itself when the family goes to the Sumiyoshi Matsuri at the Sumiyoshi Taisha shrine. While the shrine maidens were performing the Kuma no Mai to purify the area, Kenta pulls Yukiko aside and says he's been hearing rumours of a demon in the area that comes out when the fog is thickest at night. He wants to challenge it. Whoever gets the kill gets to keep their ancestor's blade. Yukiko thinks it's an awful idea, but she just wants to make Kenta happy, so she goes along with everything he does, including this. They sneak off to go looking, but they don't have to go very far before they encounter it. 

Yukiko's inexperience and Kenta's lack of skill prove fatal. Within minutes, Kenta is sliced in half, and Yukiko is next on the chopping block. Luckily for her, she just barely avoids death because Yukimura noticed his children were not with him by the time the Sumiyoshi Odori had started and ran to their aid. He kills the demon, but is so disappointed in his children fighting over this blade that he takes it from Yukiko, and makes her carry Kenta's instead as a mark of shame. 

Their relationship is strained. Nothing is good enough for Yukimura anymore. She receives no praise from him, and he holds Kenta's death over her head as if she didn't already blame herself entirely for not being able to prevent it. Totsuko does not speak very warmly to the both of them. She barely speaks at all after this. She knows nothing could've predicted it, yet she cannot help blame her husband for pushing their children too hard, for introducing them to the world of demon slayers in the first place, and for his negligence, and she blames Yukiko for not being strong enough to save her brother. She asks Yukimura if she can return to her estate in Ōsaka, and he allows it.

Without Totsuko in the picture, there's nothing left to do but train Yukiko harder. It culminates in a fight between the two of them, where she attacks her father in her aggravation, crying and yelling about Kenta being prideful and haughty and arrogant and cruel. All he ever did was mistreat her, regardless of her love for him. It was his own damn fault that he died, he deserved it. It was her father and mother's faults too, because they never stopped him, or saw what he was becoming. She loses against her father. Instead of getting angry at her, he tries to apologize, but she  can't hear him. She doesn't want to listen to a word he has to say, and she hides in the room where the maids sleep.

It's not too long after that she leaves the estate to take the Final Selection. 

During the trial, she meets a young man who is older than her by only a few years named Seiichiro Ishima, who flirts with every female competitor around him, Yukiko included. He treats the Final Selection like it's one big party, and she worries he will be among the first ones to die, but as soon as it starts, his entire demeanour changes, and he promises her he will keep her and everyone there safe. When they are separated, her thoughts stray to him. This distraction lands her in trouble when she encounters a large demon who quickly overpowers her. Though she is prepared to end her life before the demon can swallow her, Seiichiro appears from nowhere and slices the thing's head clean off. Instead of berating her like she expected, his hands are warm as he pulls her to her feet and sheathes her blade, and then he's gone again, off towards someone else's screams. Unfortunately, as hard as he tried, he could not save everyone, and he carried this burden on his shoulders as he and Yukiko stood side by side to receive their ranks and Kasugai crows. 

As she leaves on her first mission, Seiichiro excitedly catches up to her, stating that they were heading in the same direction and should keep each other company. It doesn't take long for them to become close friends, and they run into each other plenty more after that.

Notes:

Notes: 

STATS

Full focus breathing: This skill is used to generate an oxygen supply in the user's blood and strengthen their muscles, and gives them the power and stamina to combat demons.

SILENT STEPS: Those training to use the Breath of Silence are first trained to be able to walk and attack without making a single sound.

Yukiko's blade is designed by the swordsmith Kanetsugu Fujimaru (鐘続 藤円 — "bell, chimes," "next," "wisteria," "circle"). When Yukiko first exposed her blade to sunlight, it turned a shimmering powder blue colour. The golden handle is decorated with a crescent moon on the end, with a dark blue tassel hanging off of it. The sheath is a similar magenta colour to Yukiko's eyes, and painted with dark purple water lilies.

Breath of Silence

A breath style passed down through the Higashihara bloodline, it is a distant derivative of the Breath of Thunder, though it is unknown if the progenitor, Ishizu Higashihara, had any relation or any training from Yoriichi Tsugikuni, and it does not appear to be taught out of failure to learn the Breath of Thunder.

Described as the moment of silence between a lightning strike and the rumble of thunder, this breath style utilizes quick, precise movements to isolate a target's opening and aims for the spaces between an enemy's attacks.

First Style: Nothingness (壱の型虚無, Ichi no kata: Kyomu) — Also can be read as "negation;" the user stops a physical attack dead in its tracks, redirecting the momentum back into the target in full, blowing them to bits. Timing is everything with this style, otherwise the user risks breaking bones or even having their arms ripped clean off.

Second Style: Penetrating (弐の型貫通, Nii no kata: Kantsū) — The user pinpoints the spaces between attacks and jabs their blade through these spaces faster than the eye can see.

Third Style: Clearing (参の型広場, San no kata: Hiroba) — The user clears their mind, and looks for the weakest point in a target's attack, striking at the very moment it presents itself. This style is useful for breaking through barriers.

Fourth Style: Deafen (肆の型聾する, Shi no kata: Rōsuru) — The user strikes a target at the height of their attack, and the resulting force deafens the target for a few minutes afterwards, as well as messing with their balance and giving them vertigo. Timing is everything with this style, otherwise its effects are turned back on the user.

Fifth Style: Reverberation (伍の型残響, Go no kata: Zankyō) — The user deflects attacks coming towards them, using the attacks as a jumping off point to pinball their way through the spaces in a barrage and land a devastating strike on their target.

Sixth Style: Oppression (陸の型弾圧, Roku no kata: Danatsu) — The user slashes right through a direct attack, completely dissipating the oncoming attack, as well as delivering death to a target.