Koyuki was a frail, fair-skinned young lady with large, rounded pink eyes with white pupils shaped as flowers. She had black hair which was always tied into a large bun fastened with a hairpin and decorated with a Kushi (hair comb) and three snowflake hairpins.
Koyuki wore a circle-patterned, pink kimono that faded into a snowflake pattern tied with an obi.
Koyuki had a kind and gentle personality, showing much gratefulness to Hakuji during his time looking after her. She would get emotional and cry when having normal conversations with him, especially concerning his reassurance of returning to the annual fireworks show, as it reminded her of hope and that there were many opportunities to enjoy the things she never had the chance to enjoy.
Koyuki was born very sick and with a weak body. As a result, her mother drowned herself as she did not want to see her baby die. While she was ill, her father brought home Hakuji and introduced him as her new caretaker, allowing the two of them to become acquainted.
Later on in her life, the heir of the dojo next door fell in love with her, but he had an extremely violent and arrogant personality. He could never sympathize with her illness and during one meeting, he forced her outside when she was in bad shape. She started having an asthma attack and he abandoned her out of fear. Hakuji later discovered her alone and in pain, and potentially saved her from dying.
Koyuki was able to heal and live with the help of Hakuji and her father, she recovered two years later and became engaged to Hakuji.
Following Koyuki's marriage, the heir of the dojo next door became enraged and gathered his disciples to fight Hakuji. However after failing to defeat him, they cunningly poisoned the water well at the dojo. Koyuki and her father were poisoned after drinking water from the well, and she passed away at the age of 16 years old on the same day as her father.
Though he doesn't recognize who she is, Koyuki persists in trying to stop him and causes him to question his reasoning for wanting to kill the two swordsmen. As a result, Akaza remembers his memories of her when he was a human.
After regaining his compassion, Akaza attempts to destroy himself in order to finally reunite with his family. He tries to stop himself from regenerating but Muzan attempts to gain control of his mind. As Akaza begins to submit to Muzan's control, Koyuki's spirit gently grabs hold of Akaza's face and convinces him that he has done enough. Koyuki tightly embraces him and cries as he tearfully apologizes to her for failing to protect her.
She welcomes him with love and expresses her relief at his return home. Their spirits then disappear in an inferno of flames as Hakuji accompanies Koyuki to the afterlife. 
Koyuki was an ordinary human who did not posses any abilities or powers.
- Her name contains the kanji for "love" (恋 ko?) and "snow" (雪 yuki?).
- Akaza's characteristics as a Demon are closely related to her appearance:
- Akaza's hair was the same color as her kimono, pink.
- When Akaza activates his techniques, the snowflake shape of the formation resembles the hair decorations Koyuki wore.
- In Kimetsu Academy, Koyuki is a first year student and a member of the handicrafts club. From the time they were kids, she has been promised to marry Hakuji Soyama . She is his neighbour and their parents agreed to it. Their nicknames are Hime and Hakuji-Dono.
- (To Hakuji) "Those silly talks with you Hakuji, made me happy. "
- (To Hakuji) "I... couldn't imagine living another year or another after that. My mother was that way too. She didn't want to see me die... I believe that's why she killed herself. And I had knew my father had given up. I was so terribly weak. But you could see my future... as if it were just a matter of fact. You talked to me about next year and the year after that. I was very happy. Yes, you are good enough. Will you marry me?"
- (To Akaza) "Thank you Hakuji, You've done enough."
- (To Hakuji) "Welcome home my love."
- Kimetsu no Yaiba Manga: Volume 18, Extra Pages.
- Kimetsu no Yaiba Manga: Chapter 153 (Page 19).
- Kimetsu no Yaiba Manga: Chapter 157 (Page 1).
- Kimetsu no Yaiba Manga: Volume 20, (Extra Pages).
- Kimetsu no Yaiba Manga: Chapter 155 (Page 11).
- Kimetsu no Yaiba Manga: Chapter 155 (Page 12-13).
- Kimetsu no Yaiba Manga: Chapter 156 (Page 18).
- Kimetsu no Yaiba Manga: Chapter 156 (Page 20).