Demon Slayer: I Lost My Memory from the Start and Created My Own Breathing Style

[Demon Slayer: Kimetsu no Yaiba, transmigration and rebirth, genfic-focused (more or less), trying to make up for regrets]

(One more thing: The main story is genfic-focused. The subplot, namely the pre-rebirth girlfriend who appears in the heroine's dreams, is a romance pairing. Their storylines develop alongside each other, but the subplot gets less attention.)

Her soul transmigrated into the body of a six-year-old girl named Ishihara Oriya. When she woke, she had lost all her memories from her previous life, and struggled to survive in an era filled with demons.

At twelve, a demon attacked the village where she lived. On the brink of death, she was saved by Shinjuro Rengoku, the former Flame Hashira. She studied Flame Breathing under his tutelage, but her nature was too calm for her to master its breathing rhythm. On his recommendation, she became a student of Sakonji Urokodaki and learned Water Breathing, spending her days alongside her senior student, Makomo, as they trained in the Breathing Style together.

During their days of training, she combined the stillness of Water Breathing's flowing currents with the fierce heat of Flame Breathing, along with her own melancholy and compassionate nature, and created Ash Breathing (Jin no Kokyu) for herself.

After being knocked unconscious while fighting alongside Kanae to drive off Doma, she regained her memories of her previous life during the three days she remained unconscious.

(Afterward, she kept searching in her dreams for the girlfriend she had before being reborn, finding a brief respite in doing so.)

She swore to rewrite the fate that had been set for her and her companions.

Because of her lost memories and past experiences, the heroine has grown used to being silent and distant. Only when a familiar face appears in her dreams does she learn that walking side by side with someone isn't frightening after all, and understand what it means to have companions.