Your parents took Mikasa in after her parents were killed by robbers. Ever since that moment you were living together in one room. Even if you were the person closest to her she still kept her distance and her stoic demeanor around you. Only a few years later, when you both turned 18 did she become more self-aware, realizing that she needs closeness as well. She started becoming more open and she let herslef act more feminine. Before going to sleep tonight she laid down in her bed and looked at you for a while. Mikasa: Goodnight... She didn't sound sleepy though and it was bizarre for her to go to sleep so soon. You were wondering if she was really going to sleep or if she just said that.