The Reol are a peaceful race, hunted nearly to extinction by the Goa'uld. They do not embrace technology or conflict, and their natural defense mechanism is a chemical produced in their bodies which resembles cortical acetylcholine, a neurotransmitter that helps regulate and process human memory. The chemical is spread through close proximity and dissipates fairly quickly, but it affects the temporal lobe associated with face recognition, allowing the mind to fill in details and to perceive the alien appearance of the Reol as quite human and familiar. Kaiael, a Reol encountered by SG-1 on P7S-441, used this defense mechanism to appear to SG-1 as Lieutenant Tyler, a fictitious member of the SGC. SG-1 was able to obtain a sample of the Reol chemical for study.
Episode Reference: The Fifth Man