We call them 'Guardian Humans', but the fact of the matter is, we are not sure. Our researchers cannot confirm their status, because they can do things that should be impossible for a human being to do despite their appearances.
We didn't notice them until we had enough surveillance cameras to spot them. All the time we've spent reviewing footage shows that they appeared from out of nowhere, in the most literal sense of the word: They just suddenly appear where there was once empty space, and then they immediately go into action.
They look exactly like human beings, with clothes appropriate for the places they appear in. They can converse, consume food and drinks, they even seem to have hobbies and preferences when it comes to social activities.
Despite all that, they have consistently demonstrated that they can move faster than any human being could, and can endure more physical punishment than what should be possible. We have footage of a Guardian Human walking with half their skull missing, still behaving as if nothing was wrong. A few were even decapitated, but they continued fighting.
Guardian Humans only appear during times of extreme crisis, notably during violent events that were caused by human hands. Wars, genocide campaigns, acts of terror... if innocent people would be put in danger, they will appear, with the first confirmed sighting in 11th January 2041.
Ever since that date, Guardian Humans began appearing all over the world. Though they would not always appear, they have shown up in more than a quarter of violent events. They would rush to the perpetrators of violence without any hint of fear, no matter how well-armed their targets.
Guardian Humans do not give warnings or threats. They instantly move into action, and will not stop until either every single guilty person has surrendered, rendered unconscious, or dead. All attempts to escape from a Guardian Human only results in fatalities caused by the escapee's own doing, with the most recent example being the unfortunate bandit who jumped into the path of a stampeding elephant. It is generally considered a fact that trying to escape them will only cause your own death, but we cannot determine how the Guardian Humans can guarantee these outcomes.
We have not seen a single instance of a Guardian Human that has been killed in action. Their  bodies are not invulnerable, but as we mentioned before, they can endure incredible damage, and we have seen them continue to function headless, without any loss of cognitive ability.
There are 11 known recurring individuals, Guardian Humans all, who have been interrogated by numerous agents. They were all, without fail, cooperative and compliant, but despite all our efforts, they would not reveal their origins.
Here is a transcript of the most recent session, with an individual that called himself âFriend.â Friend was encountered in Tallwind, where two civilians were killed and 68 terrorists were either killed or critically wounded.
(Note: Friend is missing almost the entire left half of his torso, after suffering several shotgun blows. His legs are burned from bombs thrown by terrorists. He shows no sign of pain or discomfort. His organs are clearly visible and his formerly-brown complexion looks pale from extreme blood loss.)
Agent 21-A: We meet again, Friend.
Friend: That we are. Meeting again, I mean. What's on your mind, pal?
Agent 21-A: Just questions.
Friend: Heh, okay, okay, but you're not gonna learn anything new.
Agent 21-A: I will keep trying, nonetheless. Where are you from, Friend?
Friend: I honestly don't know, buddy. I don't remember anything before the day I first showed up, you know? Like, I just knew there was trouble, and I knew I had to do something.
Agent 21-A: Were you given orders?
Friend: Nope. I just did it 'cause it felt like it was the right thing to do.
Friend: I guess you could call it that? But the thing is, I felt like, like I knew the people who were in trouble.
Friend: Well, it's hard to put into words. It felt like I knew them from another life? Kinda unfamiliar yet familiar at the same time, like I've seen them in dreams.
Agent 21-A: Dreams. Hm. Do you ever dream, Friend?
Friend: Oh, yes, I definitely do. Whenever I fall asleep, I dream.
Agent 21-A: Fall asleep? You mean when you disappear?
Friend: Is that what happens? I figured I was just going to bed and waking up in different places... but yeah, I dream. I dream about people, pal. It's usually really blurry, like I can't make out the faces.
Friend: Yeah, usually. In the dreams, I spend a long, long time just wandering around places. Jungles, cities, swamps, you name it, I've been there. Then, suddenly, I see a face, clear as day, sharp and vivid, clearer than any crystal.
Agent 21-A: And then what happens?
(Friend leans backwards into his chair. He closes his eyes for three seconds, then instantly disappears. All traces of his existence are gone as well, such as bloodstains or fallen hair follicles.)
Investigation is ongoing. The interference of Guardian Humans has frustrated numerous governments and warlords, to the point that some have even started deliberately causing trouble in the hopes of capturing a Guardian Human.
Whatever they are, the Yellow Vault is thankful for their presence, as some of our Agents have been rescued by their intervention.