Outsiders are mysterious beings, not quite true life forms, despite outwardly appearing sapient. They are typically found aboard smaller UFOs, early in XCOM's conflict against the aliens.
When first encountered, the Outsider appears as a luminous orange crystal, which quickly generates a bronze humanoid body out of pure energy, presumably using hard light. It defends itself using a light plasma rifle, which it is capable of using to deadly effect, though it prefers to fight defensively as it has no other means of attack. Examination of the Outsider's crystal core indicates that it functions as a sort of relay construct, receiving instructions from an alien commander elsewhere on Earth and relaying them to alien units it is attached to in UFOs.