Stephen Terril, also known as Jonathan Rex, was the original owner of the Terror Castle, a star in silent movies, who lisped, and talked with a squeaky, high-pitched voice. He used to act mainly in horror movies, and even built his house to look like a haunted castle set in one of the movies in which he acted. As he could change his face in any of the superatural creatures, he earned the nickname, 'The Man with a Million Faces'.
Many years before he became a movie star, his shyness and lisp made it difficult for him to handle his business affairs. To remedy that, he created the persona of Jonathan Rex to be his business manager, who spoke in a fierce tone, that hid Terril's lisp, and due to Rex looking meacing, he had no problem in dealing with people. No person, except his best friend, Charlie Grant, knew about his alter-egos. In fact, Grant was his make-up man, who helped him change from one persona to the other. As Jonathan Rex spoke in a menacing and sinister whispering voice, he earned the nickname, 'The Whisperer'.
This scheme worked until the time when he acted in his first talking movie, it was discovered that he had a lisp, and talked in a squeaky, high-pitched voice. He was humiliated and couldn't find any more roles; his career was ruined. He "sent away" all his servants, and his supposed best friend and business manager, Jonathan Rex. When he learnt that the bank had to take his home as mortgage, he became despondent and desperate. Before this, during the time of the building of his castle, his workers had actually descovered a fault in the rocks of Black Canyon. The fault ran through the ridge to the other side, where Winding Valley Road ended. He had walled the tunnel, and had also secretly bulit a hidden door. Then, as Jonathan Rex, he bought the land at the other end of the passage and built a small house there. Thus, he could come and go to Terror Castle without being noticed.
Once, while going for a "long and solitary" drive to "shake off" his depression, he got an idea to fake Stephen Terril's death. So, he wrote a letter, and in one dark and stormy night, he staged the death, letting his car topple over the cliff.
Although the castle was empty, he could come and go with the secret tunnel. He was secretly in the house when the police conducted their search, and with the various devices he had installed for giving his friends some "thrills", he was able to scare them, and they "left in a hurry". He, later, used these devices to convince people that the "castle" was haunted. While all this, he saved money, as Jonathan Rex, a breeder of rare pet birds, for buying the castle himself. He claimed that he had acquired almost enough money to buy it, before the sleuths came to investigate it.
The day The Three Investigators came to investigate the 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. He immediately went to his house, and was successful in scaring Jupiter and Pete. After he returned to his bungalow, he searched for the sleuths, and found their phone number, and warned them once.
After Pete and Jupiter escaped from the cave they were trapped in, Grant was able to hear them give Terril's, or Rex's, address to Worthington. He contacted and told Terril about this, who, in the persona of Rex, made a fresh pither of lemonade, and lied that he had been cutting dry bush. He, then, tried to impress them with the frightening quality of his house, so that they wouldn't continue to investigate it.
After Jupiter and Pete had almost solved the mystery, Grant and Terril, using Terril's wigs and costumes, tried to give them a scare, by acting as gangsters who were occupying Terror Castle as a hideout. However, they both were about to free the boys, before Worthington and Bob rescued them.
Later that day, when the boys and Worthington came to talk to Jonathan Rex, he, along with Charlie Grant, revealed all the secrets.