The use of a main character is neccessary in any story, so that one may have a point of view for basis. This point of view allows the reader to be more knowledgeable about what is going on in the character's head. It gives insight to thoughts and feelings of the character and lets the reader know when the character is feeling pain or is just plain exhausted. It would be unnecessary to have more than two main characters to be used as reference and it is really more difficult for the reader to be able to distinguish between the thoughts of more than one person, let alone more than two.
Readers must be able to locate a happy medium when reading and writers have to be able to provide that happy medium. In classic detective writing, it is important to create a main character, the detective. The detective can be formal or informal but it is important to establish his attitudes and morals. The character is almost as important as the story that it helps to tell. If a character was produced that had no background, the reader would not understand completely why the character did what he did. Common phrases of the character are also helpful to have on hand because that is a trademark of the character. Think of Sherlock Holmes, what is one phrase that is best remembered of Doyle's oh so popular detective? "Elementary, my dear Watson, elementary." People commonly use that phrase even now, in this day and age, in a kind of silent homage to the character. Despite the fact that Doyle never actually had Holmes utter that phrase.
This detective can be crazy or just a little off in the head. It doesn't matter as long as the character is well brought out and foreshadowed. Such characters must be able to be imagined by the reader so that they may hold the reader's attention during even the slowest period in the book or short story. There is no supernatural power that can be used by the character because that is completely against the rules in the classic detective story. These stories are modest, simple tales of crimes that have been committed by someone unknown. The crime must be solved and the detective must be the hero at the end of the story. The detective cannot, however, commit the crime that is being investigated. No matter how interesting it may seem to put that into the story, again, it is against the rules of the classic detective fiction story. The character must be winning, genuine, and have minor quirks that normal or slightly abnormal individuals have. Make the character as real as it can be.
If the writer chooses to make a second main character, this character should be able to flow evenly with the other main character. Such is the use of Watson in Doyle's tales of Sherlock Holmes. Although Watson may not be a main character exactly, he is definitely a main supporting character whose thoughts and feelings are aware to the reader.If one can effectively create a winning character and a winning storyline, the rest will be history.