A thin man

Throughout their journey, Mono and Six encounter televisions that are turned on, which negatively affect Mono. When Mono tries to tune in, he's transported to a long corridor, at the end of which lies a locked door. However, no matter how hard Mono tries to reach it, his attempts end in failure until he opens it a third time, revealing the Thin Man sitting on a chair.
As Six pulls Mono away from the television, the Thin Man follows them out. The children try to hide from him, but Six fails, causing him to find her, grab her, and then vanish with her, leaving only a staticky silhouette.
Some time later, when Mono finds a television with Six trapped inside, he tries to help her, but the girl is immediately grabbed by the Thin Man and carried into the depths of the screen. He emerges and begins slowly following Mono, pursuing him through the apartments of yet another apartment building. The boy eventually escaped by jumping out a window.
Having escaped the building, Mono finds himself on the local train tracks, where the Thin Man quickly finds him again. Continuing to flee from the monster, who pursues him with calm and confidence, the boy manages to escape on a train car, uncoupling it from the rest of the train. However, upon arriving at the station, the car stops abruptly, nearly killing Mono, who is severely injured. After a moment, the boy overcomes the pain and moves forward, following Six's Chaotic Remnant, who leads him to the stairs leading up to the street, where, in front of the Black Tower, he is once again confronted by the Thin Man. Mono removes the bag from his head, stands up abruptly, and begins to fight, unleashing a stream of energy at the monster. However, Mono proves stronger, and the Thin Man, having fallen once again, is unable to rise, no matter how hard he tries, and then simply vanishes into thin air. At the end of the journey, when Six abandons Mono during their escape from the Tower, the boy ends up in the Thin Man's room.**