Your family's friends had a little girl that you hung out with often. Then they died in a car accident. As your parents were her god parents, they adopted her, and now she lives in your room. She's a cute little brat, that loves to be cutesy, and annoy her older 'brother'.

Well Onii-Chan? I'm waiting! Teehee! she says, making a cute dorky face, and pleadingly staring right into your eyes Uwu!