Disclaimer: I do not own the Harry Potter series!
Katherine knew Tom Riddle from the time he came to hogwarts. They were both in Slytherin.
She never knew he was this serious about everything he talked about; he could be so secretive. He wanted her to kill a particular muggle-born. She refused, his growing need for power was alarming and frightening. When she refused, he had smiled at her and whispered in her ear that she was the most beautiful pureblood he had ever laid eyes on. She replied, That may be but I won't do this Tom. He sneered. He demanded he be called Lord Voldemort now; she refused. He would always be Tom to her.
They argued quite a bit, this time though his anger gleamed in his eyes. This time he couldn't be reasoned with. He grabbed her by the arms. She fought. He was too strong, too powerful. He bent her over, slammed her into the desk and ripped her skirt down. Tears ran down her cheeks. He thrust into her as he pressed her into the desk. She was dry and it burned like hot coal. No more than four violent thrusts and he grunted. She could feel his warm seed between her legs, it felt vile. He yanked on her blonde hair and muttered that she had better learn to do what 'Lord Voldemort' said.
Katherine went to the find help from the only person she felt she could trust, Professor Dumbledore. Katherine asked Professor Dumbledore if anything should happen to her to give the child to a family that would love them. She told Dumbledore who the father was and what had happened nine months ago; she requested that no one know and assured him that Tom wasn't aware of her pregnancy.
In the beginning of December 1959 Dumbledore sadly handed a beautiful newborn baby girl named Arianna Katherine to a loving young couple Eric and Angelina Mullen.
Eleven Years Later...
The hat was placed on her dark wavy hair. The hat stated that Slytherin would indeed be a good fit for her but it paused contemplating then shouted Gryffindor. She glanced at Dumbledore, her dark eyes glimmered. On certain angles he could see her mother, but there was no mistaking in his mind who her father was. The resemblance was remarkable.
She introduced herself to a boy sorted into Gryffindor moments before, Sirius Black.