Helena Ravenclaw
He was kind to her; he listened and made time and didn't avoid her like every other living witch and wizard in the castle. At first she'd tried to avoid him but he had been determined and resilient in getting her to warm up to him. Looking back on that, it should've been her first clue that everything wasn't as it should be.
He spoke softly to her, telling her about his day and some of the bafoons that were in his class. He never spoke a terribly bad word about anyone. Eventually she grew used to his blabbering and started to respond. Oh if only people knew that she'd responded to a Slytherin. The Bloody Baron would throw a fit and Ravenclaws would be filled with disbelief. It was unknown for her to converse with anyone other than members of her own house. Occasionally she spoke to other ghosts but there wasn't much company for the dead so it only made sense.
He was the first living person to hear her secret; the only person to hear it. She regretted it almost immediately.
He seemed to take on a different persona, one that made her fill with dread and she knew that if blood rushed through her veins, she'd have paled dramatically. His Slytherin side didn't come to show until he returned a few years later, the precious diadem that people hold so highly, in a box. He held it gently and lovingly but even from her position at the end of the corridor, she could sense the dark magic that filled it.
He'd defiled her mother's diadem. And it was all her fault.