Charlie was a genetically-engineered human boy designed to communicate with the SGC by a Reetou geneticist referred to as "Mother". He was introduced in the episode, 2.20 "Show And Tell".
Charlie was designed by a Reetou geneticist to communicate with the humans of Earth and tell them of the plans of the Rebel Reetou to kill all possible hosts of the Goa'uld through genocide. The Reetou are an insect-like, intelligent alien race who live 180 degrees out of phase with the humans of Earth and are, therefore, invisible to the naked eye. The Tok'ra used a Transphase Eradication Rod (TER) which was developed by the Goa'uld to bring the Reetou into the visible spectrum and kill them. Charlie, however, had been engineered to see the Reetou Mother with his own eyes.
Mother had come into the SGC prior to Charlie and observed their behavior. She was present when the Salish Indian Tonané had visited from his homeworld (2.13 "Spirits") and found Jack to be an honorable and trustworthy man. Charlie connected with Jack and asked that he be called "Charlie" after Jack's son. This request saddened Jack, but he permitted it. The two of them developed a close bond.
After Charlie successfully communicated the warning about the Rebel Reetou, the SGC and Tok'ra developed a way to detect any Reetou who might come through the stargate. Additionally, the SGC installed a palm scanner to control the iris protecting the stargate because Mother was able to manipulate the keyboard to input the commands to open the iris for Charlie when he first arrived.
During a Rebel Reetou attack on the base, Mother was killed. Her death was witnessed by Charlie. He wanted to stay with Jack, but his health was quickly deteriorating. His accelerated growth didn't allow him to mature correctly and he was dying. Jacob/Selmak took Charlie to become a Tok'ra host so that he could be healed. We never hear of him after this, although Jack did promise to visit him.
--DeeKayP 06:50, 20 Apr 2005 (PDT)