Kaiael is a Reol, a race that protects itself by producing a chemical that can be passed on by touch and which causes one to feel a sense of recognition, thereby disguising the Reol's true appearance. Kaiael had been captured by the Goa'uld for study, but escaped and crashed on P7S-441, where he encountered SG-1. He used his defense mechanism to appear to them as Lieutenant Tyler, a member of SG-1. He and O'Neill were trapped on the planet, cut off from the stargate by Jaffa patrols, while Carter, Daniel, and Teal'c returned to the SGC for reinforcements and faced interrogation concerning their insistence that Lieutenant Tyler was the fifth man on their team. Realizing that his means of self preservation had placed the lives of others at risk, Kaiael revealed his true identity and appearance to O'Neill, but SG-1 refused to leave him behind. Following their rescue, Kaiael returned to his people, to a new colony on a planet in an isolated part of the galaxy, and a sample of the Reol chemical was retained for study by the Tok'ra and Area 51.
Episode Reference: The Fifth Man