Martin was possessed by the Ancient Lights. He began to give highly accurate readings at a local theatre, quickly gaining popularity. He started a show in which he demonstrated his talents before a live audience. His act caught the attention of the Psychic Channel, which gave him a medium to enact the Ancient Lights' master plan on a nationwide scale. He also took an interest in Sarah Jane Smith, knowing she had travelled with the Doctor.
The power of the Ancient Lights enabled him to fire energy blasts and exert control over his clients, as well as give accurate information about the past and future.
When his plan of possessing the world and making them "join the circle and the Ancient Lights" was foiled by Luke Smith, he said it was his destiny and disintegrated into space, allowing the Ancient Lights to make him "one with the stars". (TV: Secrets of the Stars)
When he was a con artist, Trueman was a cynical man who did not believe in astrology, yet pretended to be knowledgebale on the subject in order to trick people out of their money. He seemed to possess some morals however, as he refused to accept Cheryl's mortgage when he learned she was paying him with it, leading him to confess his decpetion to her in shame.
When he was possessed by one of the Ancient Lights, Trueman became inspired and embraced his position as the 'Chosen One' he developed a strong desire to inspire others along with fufilling the ancient lights purpose, he also developed a strong confidence, believing that he was center of the universe he even believed that the Ancient Lights were working for him despite being grateful to them for bestowing him with their powers.
The case of 'Trueman the astrologer' was one of several alien incursions which occurred in the "Ealing Triangle", cited by Ruby White when she met Sarah Jane and her team. (TV: Goodbye, Sarah Jane Smith)