SCP-001, also known as "The Gate Guardian", is a Euclid/Keter class humanoid entity that is one of many potential entities that share the SCP-001 title. It can be viewed as the overarching protagonist of the entire mythos. SCP-001 is the being that prompted the SCP Foundation to exist. He said one word that echoed through the administrator's head, "prepare". It was that one word that made the administrator create the Foundation for an unknown future event, and to contain many other dangerous or helpful entities and anomalies. It has never left its station.

It is theorized that the Gate Guardian may be the biblical angel Uriel, an angel that guards the gates to heaven, yet it is not confirmed.