The Battle of Hogwarts claimed over fifty lives. It destroyed the castle and many students would never return, many professors would never return to teach. Voldemort vanquished by Harry Potter's hand, the Death eaters either captured or fled, life was starting to be rebuilt. The castle itself was remade, students who survived the battle returned and new ones attended. New professors have taken their places, the world starting anew. For nine years, things of this world have been repairing the damage Voldemort had done.
Nine years ago, Arno Rigby's parents died at the Death Eater's hands, his mother died at the hands of Lance Redhart. He saw it himself; he'd been a seventh year who volunteered to fight. His parents refused to let him fight alone, though they were proud of him. He knew it, he came from a very close family. Something that he knew that he was very lucky to have, especially in times like he'd grown up in. Watching his mother die that day was the worst thing that he'd ever experienced. It was now that he saw what pulled the carriages heading up to the school; grotesque horse like creatures called thestrals. He understood them, and death a little more after researching about them. He didn't know who had slain his father or what terrible end Fredrick Rigby met, but of this he did know about his father; he was a great man.
Arno often wondered whether or not that his parents would be proud to hear that he'd become a professor at Hogwarts. But whenever he wondered, he always knew that he was being foolish. His parents loved seeing him excel in his favourite subject; they would have been so proud to see him get the job he'd always dreamed of. The school year would start soon, the professors had arrived months before the first student would step onto the grounds. He'd been made the head of his house, lived in his own quarters and met the rest of the staff. The Potions Master still hadn't arrived yet, but he knew who she was. A fresh wave of hatred rose in his mind as he sorted out the papers on his desk. He knew that he shouldn't hate her, it wasn't her after all that had killed his parents. No, she was their daughter; the two had never gotten along. Ever since childhood as classmates, the two had hated each other. Now, he had so much more of a reason to. Even after nine years, his parent's deaths were fresh on his mind.
So, there he sat, writing down his first lesson for every class. There were a few students that he knew of that sympathized with him. Many students were orphaned by the Great Battle, others before. All professors were trained to talk to suffering students and other faculty. Nine years wasn't enough to heal deep wounds.