FSDSS-524 A young man takes revenge on someone who bullies him at school

The story's main character, Yuki Yudzuru, was heavily bullied in middle school, and he eventually became an introvert, spending all his time in his room. Some time later, his father remarried a woman who already had a daughter, a girl named Koyomi, who is now considered his new stepsister. But to his shock, he discovered that this was the same girl who had bullied him. Yudzuru reacts very badly to this news, even worse when Koyomi leaves him a very aggressive letter: calling him a virgin and telling him to never leave the room. So, Yudzuru decided to find her weakness and take a suitable revenge for her.