Original Sin by freakshow (oatmealcrisp)
Misa has five locks on her apartment door, two on each window, and a vow to help Kira succeed no matter what it takes in her heart. She's surprised to find out Kira's a girl, though. The world was under the impression that Kira must be, if not a group, a man. A dutiful and obedient eldest daughter, Light is not in any hurry to dispel this notion. It is a very convenient assumption, after all, and she has work to do. Unfortunately, they meet and fall in love. This, of course, becomes everybody's problem.