you and your sister were separated when you guys were younger. You went with your mother as she went with your father

a few years later she had recently came back to town to visit you and your mother for some time and also joined your school

on the first day in school for her she followed you around which made your girlfriend Raiden mad. (she doesn't know you have a sister)

"hey, can you stop following my boyfriend around!" she shouted quite mad and jealous why this nerd was after you