"Happy 10th Birthday, (Y/n)" everyone from the infinity castle greeted.

It's been four years since (Y/n) lived in the infinity castle with Muzan and the upper moons. The upper moons didn't like her at first, but now, they were celebrating her birthday together and adored her so much.

"Make a wish," Akaza said. (Y/n) closing her eyes, thinking about her wish.

"I wish that I could be with dad and the upper moons forever. I hope that they will always love me because I love them too" she prayed and blew the candles.

Douma couldn't help to tear up "That is so sweet, little one," he said while covering his face with a fan.

Muzan sat beside her and hugged her "Of course we can be together forever. I will always protect you and love you" (Y/n) hugged back and smiled at him. "Me too, dad"

"This is my best birthday ever! I hope we can be always like this forever"

Later on, they proceed to the celebration by spoiling her expensive presents.

They celebrated until she got exhausted, leaning on Akaza's lap. "Why Daki and uncle Gyutarou didn't come on my birthday?" she mumbled and made everyone surprised by that statement. "I haven't seen them and I miss them"

The truth is, the siblings got killed before the day of her birthday. Muzan chose not to tell her because he doesn't want to ruin her excitement.

Muzan shifted her head to his lap "Sweetheart, something came up to her village. But, she handed me this present for you" He gave her a cat hairpin. "Pretty. I hope she will do my hair again" then she drifted to sleep.

Muzan carried her, walking to her room. He laid her down on the bed and wrapping a blanket on her. "Happy Birthday, sweetheart" he whispered.

"I will always protect you even it will cost my life. I don't know why am I like this to you. But I'm glad that I am your father and you are the only one who makes me happy.  I cannot believe that we get along so much to the point that you exposed my soft side to you" he didn't realize that his tears were streaming down on his cheeks.

Eventually, he realized it so he wiped them instantly. 'What am I crying for?'

He kissed her forehead and left her room.

* * *

(Y/n) woke up in a cloudy place. It was just all blue and cloudy. Then, she heard a male voice calling her. "(Y/n)!" she walked around trying to find that voice.  That voice was familiar to her.

"(Y/n) dear!" A female voice called. She turned around and she tears up from what she saw. She ran to them and hugged them.

"Mama, Papa. I miss you so much" she cried. "Happy Birthday" they knelt to hug her back. Her mother pulled away to wipe her tears. "Mama, Papa! Someone's taking good care of me"

"He is Muzan, right?" her dad asked. "How did you know?"

"Because we've been watching you" her dad smiled.

"Are you not angry? That I'm with a Demon?" (Y/n) asked.

"We are not. As long as he taking care of you, we will not be angry" her mom smiled.

"Honey, I remembered he went to our grave alone to give us flowers"

"Small flashback"

Muzan was on his way to hold a meeting with the upper demons and before he went back to the fortress, he bought some flowers to place on (Y/n)'s parents' grave.

''Thank you for making such a wonderful and beautiful human. I promise that I would be a good parent for her''

He left after that.

End of flashback

"Dear, It's time for us to go," her dad said.

"Be happy and be a good daughter to him okay? We will always watching you"  (Y/n) nodded and they hugged each other for the last time.

* * *

Slowly opening her eyes, noticing a familiar ceiling. She realized that she was in her room. She sat up, rubbing her eyes.

(Y/n) noticed Kokushibo just standing in front of the door, gripping on his sword. "Kokushibo-san? Where's dad?"

Kokushibo turned around, seeing her awake "Can you move?" he asked which made her confused. He approached her, sitting on the bed and assuring her if she's okay. "I'm okay. Why do you ask?"

"I'll tell you later. I bet you have something to say?"

"Yeah! I dreamed about my parents. They greeted me" she hugged him and smiled. "But, I pretended that. . ." he noticed the sudden change of tone.

"I pretended that I didn't know Daki and Gyutarou died. Sorry"

"There's nothing to apologize for," Kokushibo said, wrapping his arms around her.

Later on, he handed him a mirror to let her know about her appearance. Which made her eyes widened. "Remember why I asked you if you were okay?"

(Y/n) hands were shaking, looking at her pale appearance and red eyes. "This m-means. . . I'm like you now? I'm a demon?!" she said happily.

"You are not mistaken. Muzan-Sama made you one during your sleep, so you wouldn't feel much pain"

(Y/n) couldn't stop her tears. She was about to ask if where Muzan was but then. . .

*crash*

"What was that crash? Where's dad?" she asked nervously

"Apologies, I have to leave. Don't leave here" Kokushibo quickly rushed outside, leaving (Y/n) nervous state. 'Is something bad happening?'

A few moments later, somebody just aggressively opened (Y/n)'s room. (Y/n) startled making her got up from laying down.

She looked at the door and saw Akaza. He quickly approached (Y/n) and carried her. "Akaza what's happening?"

"I got no time for explaining. I have to move you out here" he said while running in the hallways.

"Is something bad happening?" she was the urge to cry when Akaza put her down, trying to wipe her tears. He didn't know what to say. "Also, Kokushibo-san didn't answer me earlier if where's dad. Please tell me the truth"

Akaza sighed, preparing for the words he will say. "(Y/n)-chan, I'm sorry to tell this to you but, we are in a battle with the Demon Slayer Corps right now. About your dad, he is currently fighting with the Pillars"

(Y/n) couldn't stop her tears "Are you gonna leave me? Is everyone gonna die? Why are you leaving? What about our prom—"

Akaza cut her off by hugging her "Shhh, we will keep our promise okay? Don't cry" he pulled away from the hug to wipe her tears.

At that moment, a familiar guy that wearing a green-checkered cloak appeared alongside a guy that wearing a half cloak was noticed by (Y/n). "Tanjiro-Kun!" she called out.

The guy that wearing a half cloak was about to attack (Y/n) but Akaza was the one who took the damage while Tanjiro was trying to stop him by holding his wrist "Tomioka-san, please don't kill her!" Tanjiro yelled.

"Are you crazy? She's a demon! I don't care if she's a little kid. And she's Muzan's daughter! We have to finish her off!" Tomioka argued.

"I'm sure that she wouldn't harm us. I also smelled that she haven't eaten humans yet"

While they were busy arguing, Akaza was telling (Y/n) instructions about how she could escape "Be safe, Akaza-san"

"Yeah! Hurry up, your dad is waiting for you" (Y/n) carefully running away towards the room where Akaza told her.

Then Akaza fought Tomioka and Tanjiro as soon as she disappeared in his sight.

* * *

(Y/n) arrived at that room and she opened the door immediately. She saw her Muzan, they rushed to hug each other. "Sweetheart! I'm glad you are safe" he said, hugging her tightly. "Daddy, I'm afraid. Everyone is dying" (Y/n) cried.

Muzan comforted her until she calmed down. "Sweetheart, don't leave until I say so, okay?" he also explained to (Y/n) if where the rest of the upper moons were.

"Do you know, I will miss them so much. I will miss how Rui-oniisama treated me as his little sister, I will miss how Daki made my hair pretty every day, Kokushibo's cooking, the way Douma compliments me every day, Nakime's playing of her Biwa for me as a lullaby when Akaza babysits me and. . ."

"The way you protect me, spoil me and love me like your real daughter. Even though I'm a weakling, you choose to take care of me. I love you, daddy"

Even Muzan comforted her already, she still cannot stop her tears.  He also didn't realize that he was tearing up, he just hugged her again and kissed her forehead. "Sorry, If I do not become a perfect father to you, my daughter"

"That isn't true. Don't say that" they both started to cry out loud.

A minute after, (Y/n) fell asleep, and before Muzan went back to the battlefield, he wiped his tears.

During a long battle with other Demon Slayers, (Y/n) woke up. She was running to go out in the Infinity Castle. The moment she opened the door, she witnessed Muzan receiving the last blow from Tanjiro.

She ran to Muzan. "Daddy, if you are leaving, please take me with you," she said, shaking his body then she laid beside him. He hugged the little girl

"I didn't regret adopting you. I love you, my daughter" those were his final words before they both vanished.

The end for real.