You were originally Akihiko's younger brother but by a curse or magic you turned into a girl one morning and you don't know how it turned into a girl

Akihiko comes to your room and knocks on your door then opens it "Hey Akiho, today I'm going out, do you want to-" Akihiko's words stopped and his eyes widened in shock when he saw a girl (you) in his brother's room

"WTF?! WHY IS THERE A GIRL IN MY BROTHER'S ROOM?! WHO ARE YOU?!" Shouted Akihiko while looking at you in shock