In Robert Louis Stevenson's fantastical story of The Strange Case of Dr. Jekyll and Mr. Hyde, Gabriel Utterson is both Dr. Jekyll's friend and lawyer ("solicitor").
Because Jekyll and Hyde are so "closely tied," and are seen so infrequently, Utterson is a good choice to narrate the tale of what he has witnessed, for Jekyll and Hyde never present themselves to share any information. Secondly, Utterson's own lack of conclusive knowledge for most of the story intensifies the mystery surrounding the association of these two mysterious men.
Once the reader realizes that Jekyll and Hyde are the same man, one transformed to the other and back again, it is clearly evident that someone on the outside of this "experiment" had to tell the tale. In that the two characters are basically two sides of the same man, neither one would be able to tell the story without sharing the secret and indicating that both were inextricably tied to one another.
The third person in the equation, Utterson, has the details collected from various sources that he shares with the reader. Jekyll's doctor is one of these sources, and as the reader gets closer to the truth, Utterson is in possession of more pertinent information. We learn that Dr. Lanyon has knowledge that has chilled him to his very core: something from which he "will never recover." Close to death himself, he entrusts Utterson with information he must guard until the death or disappearance of Jekyll.
When word comes to Utterson that Jekyll has not come out of his room for a week, he goes to the man's home, and breaking down the door, finds a dying Hyde. With Jekyll "gone," Utterson reads Dr. Lanyon's confession. It is only with all the information now at hand that Utterson can shed light on the mysterious bond between the two men because he is an observer and our "reporter." From an objective standpoint, he not only shares what he has learned, but his level-headed approach to the entire affair makes him a credible source for this bizarre tale.