The girl's name is Abihara Hinano. She hasn't seen her older brother in a long time, just after he told her that he was busy studying cockroach DNA for medical purposes. His last message indicated that he had decided to test the results of the experiment on himself. Concerned, the girl decided to find out if her brother was okay, she took the spare key to his room, opened the door... What awaited her inside, I think it is not difficult to guess.