SCP-451 was a foundation researcher who was sent to find and retrieve an artifact. He went MIA a bit after finding the artifact and was later seen at Site 19. It was found that 451 could not see or hear humans, or see their interactions with the environment unless he looked away from the spot where the interaction happened for a few seconds. He is also immortal. He attempted to kill himself to escape the misery he went through. Instead, often someone else would get hurt. He even tried looking away from SCP-173, which didn't work. 173 moved around, but did not attempt to kill him.

FMI go to http://www.scpwiki.com/scp-451