Lt. Tyler was believed by the other members of SG-1 to be the fifth man on their team, but was really a member of the alien race called the Reol, in the episode, 5.04 "The Fifth Man".
Lt. Tyler was the identity taken by an alien whose species, the Reol, were being hunted to extinction by the Goa'uld because the Goa'uld wanted the chemical excreted by the species which allowed them to subject others to detailed hallucinations. The chemical would be perfect for infiltration since the alien was able to plant the "idea" of Lt. Tyler in all who came in contact with him and his chemical. SG-1 encountered the alien, whose real name was Yayayeii, on the planet P7S-441 where the alien was able to make SG-1 believe that he was human and had been assigned to the unit for training—thus the fifth man on the team. Jack and "Lt. Tyler" were trapped on the planet with the Jaffa hunting party and all the while, Jack protected him as if he were one of the team. When Tyler revealed his true identity to Jack, Jack still continued to protect him and SG-1 was able to secure access to the stargate for him to escape.
Tyler said that he was going to a planet far away and would bury the stargate, but Jack wanted to keep in touch. It is possible that Jack was able to keep the lines of communication open with Tyler and his fellow Reol because the Tok'ra were able to use a modified version of the chemical in a future mission involving Daniel posing as a lo'taur in the service of the Goa'uld System Lord Yu. It is also a possibility, however, that the SGC was able to give the Tok'ra the sample of the chemical taken from SG-1's uniforms by Dr. Fraiser. All that is said of the Tok'ra's research of the chemical is, "Ren Au and her team have been working on a modified version of the chemical SG-1 obtained from the Reol." (5.15 "Summit Part 1" and 5.16 "Last Stand Part 2")
--DeeKayP 17:57, 11 Nov 2004 (PST)