It was all starting with (Y/N) decided to pay a visit to her friend's house. Her friend, Yui, invited her to a small tea party in the Sakamaki household. Being a little naughty, the newly turned female vampire put some of Reiji's potion into (Y/N)'s teacup. A thick smoke covered the girl, all was left just a cute little girl sitting across Yui.
The Mukami was in deep thought, trying to figure out anything to do with the little girl. The little (Y/N) kept calling the boys as her 'big bro' and smiling at them like a little angel. After a long discussion, they came up with a decision to make Azusa take care of the little girl for the time being.
Azusa held the little girl in his arms. The girl clenched the boy's sweater tight as she snuggled and yawned in his arms. She looked at the scar tattered face of her newly found brother. She pecked the vampire's cheek and giggled.
"Azu-nii?" She called softly and hugged Azusa's neck tightly. Azusa glanced at the little girl and hummed in response. "Can I have some ice cream?" She asked innocently. Azusa was secretly melting inside, seeing his beloved (Y/N) as his little cute sister.
"Fufu~ of course... let's get... some ice cream..." he said slowly. The girl nodded happily. In the kitchen, they met the eldest son. The little girl obediently sat on Azusa's lap as she enjoyed the cold treat. Azusa and Ruki chatted for a while as the girl ate.
"Ruki-nii, dinner?" She tilted her head and pointed the opened recipe book on top of the counter. Ruki asked her if she had anything specific she wanted to eat. "Fried tofu!" She cheerfully said as the little girl climbed onto Azusa's back.
The boy and the little girl continue their adventure around the house. Playing with Kou and the kitten then helping Yuuma plucking some fresh tomatoes. Time passed by and when Azusa came back from pouring a drink for the girl, he found her sleeping on the sofa, curling like a cat which was sleeping under the orange glow of twilight sun.
"(Y/N)-chan?" He called softly after he sat beside the girl. She opened her eyes groggily and climbed to the male's lap, curled once more, she inhaled the vampire's scent.
"Azu-nii, remind me to someone I really like..." she mumbled softly and drifted off to sleep. Azusa's face flushed red. He smiled and stroked the girl's hair.
"I hope... you back soon... (Y/N)-san..." Azusa said lovingly to the girl.
Azusa woke up at the feeling of something rubbing his face. He looked at his side and found the little girl rubbed their cheeks together. Azusa stared at the girl confusedly.
"Azu-nii wakey wakeyy! Yui-nee brings something with her~!" She giggled.
The memories of her being a little girl made her blush beet red when she finally turned back into her usual age. She scolded Yui for being naughty and put the potion inside her tea.
Azusa rubbed his cheek. Having a little sister isn't bad at all.