Lelouch's Other Sister.

Her name is Elizabeth VI Britannia and she is the other sister of Lelouch VI Britannia and Nunnally VI Britannia. She's often better then Lelouch and Schneizel at Chess. After her mother dies and Lelouch takes Nunnally and are forced away, she's left alone to do nothing but train by herself. After years of training, her father the emperor of Brittania sends her on errands like Cornelia and Schneizel only she doesn't use a Knightmare or an army like your other siblings. Only on her missions does she either keep the peace or she kills the problem which inevitably keeps the peace. A few years pass and she hears of Clovis's death. She attends the funeral of her disappointing brother and begins to keep track of the events in Japan as they unfold until Zero makes his presence known.