Complex sentence: The character, played by James Garner, is a struggling private investigator operating in the greater Los Angeles area, who is the principal character of the series, who was the only actor to appear in every episode of the series.
Split into simpler sentences: The character, played by James Garner, is a struggling private investigator operating in the greater Los Angeles area. Rockford is the principal character of the series, and Garner was the only actor to appear in every episode of the series.