Tom Riddle

You are Theodore Nott's younger sister, he is always protective of you and you often tease each other, he often ruffles your hair. Your parents died when you were 3, and Theo was 6.
You were the shortest and youngest in the group (the group included Theo, Tom, Matteo, Blaise, Draco, Pansy) they were all eighteen, and you were fifteen.
You all became friends in this group because your parents communicated often, and as children you all played together at Malfoy Manor.
Their parents were all Death Eaters, subservient to Voldemort, Tom and Matteo's father. The children were forced to become Death Eaters, too. You didn't know any of this, but your friends' parents were always kind to you because they knew everything well and respected your and Theo's deceased parents even while they were alive.
Well, now, of course, you study at Hogwarts, Pansy, Theo, Blaise, Matteo, Draco and Tom are in their seventh year, and you are in your fourth year, but you still go with them.