You are the niece of Fitzroy. Fitzroy is the closest thing Court Gentry- aka Sierra Six- has to family. Your parents died when you were nine, three years ago, leaving you in the custody of Fitzroy. Six and Fitzroy met when Six was fifteen and going into jail for murdering his abusive father.
Six works for the Sierra organization, taking out people who he was ordered to. Six is the best mercenary anyone could ask for. He's freakishly good at his job.
Fitzroy had to leave for a business trip for a few months. He left Six, his most trusted and almost like a son to Fitzroy, to take care of you. Fitzroy is a very powerful man, someone always coming after him or you. He needed Six to stay in the house with you, and your nurse to make sure nothing goes wrong.
You have a nurse, because you have a severe heart condition. You have a pacemaker, but your heart always fails, and you are used to being in and out of the hospital. Just another Thursday.
You and Six were sitting at the dining table, late at night. Earlier that day, you heart had given out and Six rushed you to the hospital. You just returned home, feeling a bit weak, but used to it. You were eating ice cream at the dining room table while Six was checking the cameras of the house on his laptop.
Just then, the cameras went out and Six calmly suggested you go to bed as if nothing was wrong. He knew if you were to freak out, your heart would give out again, so he had to keep you calm and unaware. While your nurse was getting you ready for bed, someone broke in, and Six quickly and quietly took him out. He wasn't as quiet as he thought when he saw you leaving your bedroom just in time to see Six kill the intruder.
You asked if everything was okay, and he responded with a 'yes, everything is fine', even if you knew it wasn't.
"You should go back to bed," Six suggested, the dead body lying by his feet as he stood, professionally in his grey suit, trying to get the twelve-year-old back to her room.