Charlie Grant was a friend of the supposedly dead Stephen Terril, the only other person who knew about his alter-ego, Jonathan Rex (except The Three Investigators), a make-up specialist, Terril's make-up man, and the person who helped him change his personas. Later, he helped Jupiter Jones disguise himself as Professor Yarborough.
After Stephen Terril's career was ruined, and he faked his death, while living as Jonathan Rex, Grant helped him in scaring the people who came to the castle. As he lived in a bungalow, which was barely visible, near the entrance to Black Canyon, he was able to see everyone who entered the canyon. Whenever he saw someone, he would quickly telephone Terril or Rex, who could hurry through the natural tunnel to scare them. He would also roll down rocks down the Black Canyon, occasionally.
The day The Three Investigators came to investigate Terror Castle, Charlie Grant had seen the Rolls-Roys sedan enter the Black Canyon. When Grant told Terril about the sedan, Terril recognised, from the description, that it was the sedan that Jupiter had won, as he had read about it in the newspaper.
The second time the sleuths came to Terror Castle, however, him and Terril were left unprepared with the unexpected arrival of Skinny Norris and his friend. As it was too late for him to phone Terril to tell him about the sleuths' coming, he went into the canyon by a side trial, and kept watch on them. When he was seeing the sleuths chasing Skinny Norris and his friend, Grant accidently started a rock rolling, when the sleuths spotted him. Even though he had seen the boys duck into a crevice, he was worried whether they were hurt, waited there until they were safely out.
When sleuths were able to deduce that Terril was still alive, he was with "Jonathan Rex" when Rex, or Terril, revealed the mystery.