It is worth repeating that your key character is your book, and must be to the forefront of the story most of the time throughout the plot. Ideally, the key character should be introduced to the reader in chapter one. This is important, since the reader will need to feel empathy with a character straight away to be willing to follow that character through the book. If the reader cannot find this sympathy he/she will simple close the covers of the book, having lost interest. Interest is maintained by depth of character and skillful plotting.
It has been said in a previous article that the writer should know his key character thoroughly. But there is no need to outline this background to the reader immediately in blocks of explanation. Through the key character's interaction with other characters and his/her internal thoughts interspersed throughout the book the reader will come to know everything in a natural way, as we come to gradually know our friends in real life.
As the key character's background decides his/her personality and outlook on life, certain traits will emerge that will fix the character in the reader's mind and imagination. The most important trait is integrity. The key character will have ideals and principles which he/she will fight to protect and no matter what adversity he/she faces and will stand by those principles. The writer can test the key character's strength to the full here, making him/her suffer any hardship or apparent insurmountable obstacle.
Watching the character struggle to maintain his integrity, especially in early scenes and chapters, makes the reader trust and respect the character. This resulting empathy is a kind of hook to keep the reader turning the pages. Without integrity the character will not be likable - a fatal flaw, no matter how well worked out the plot is.
Integrity is not to be confused with virtue. A character may lose virtue and be forgiven by the reader, but a reader will not accept a key character betraying his/her principles.
For an example, let us look at author Lee Childs' very popular character Jack Reacher. Jack is a loner out of choice, a dangerous man; an anti-hero in fact. Jack will kill without a second thought, but the reader forgives him and still admires him because whatever he needs to do is done in the name of justice. Jack has strong integrity even though his virtue may be a little tarnished.
What other traits your character has will be determined by his background and by the plot that you weave for him/her.
But, of course, the key character is not in this story alone. There are other characters usually determined by the plot. Other characters are usually called secondary characters. These characters will interact with the key character thus pushing the plot along at a good pace. However, they are secondary by degrees. Some secondary characters will remain in the story throughout the story, having a strong role to play, and will have viewpoint scenes of their own. They also will have varies traits - some good, some bad according to their role in the story and the plotting. There are minor characters which the writer will find it necessary to bring in but they will not be given viewpoint scenes - these are what might be described as walk-on characters. They will usually have a specific function in the story if only briefly. In the case of very minor characters there is no need to name them. The reader will soon lose patience if he/she has too many names to cope with.
It is unwise, too, to create too many characters. Certainly do not bring a host of characters in at chapter one. The key character rules in chapter one, when the reader learns of the insurmountable obstacle that the key character faces, (more about that in the article on plotting.) Do not have walk-on characters popping in and out of the plot, each with their own point-of-view, then never to be heard of again.
An example of this bad practice is thus. John is eagerly waiting at the window watching for the postman to arrive with an important letter he is expecting. He sees the postman approaching and we hear his excited internal thoughts - there might even be a little dialogue as he speaks to his wife.
Now many new writers are tempted to leave John's key character's point-of-view and jump into the approaching postman's walk-on character point-of-view, giving his thoughts about this and that which has no relevance to the plot at all. Many new writers tend to scatter points-on-view around like confetti. (There will be more on this aspect in an article devoted to structure and construction in the novel)
It is best to have a very few characters the first scene in chapter one, two or three at the most. They all must have a specific role to play otherwise there is no need for them to be there at all. Talk to your characters. You may even find that one or two will try to push forward, wanting a bigger role in the story and will sometimes attempt to take over. But you know your plot and you know which way you want it to go in the beginning. But don't turn a completely deaf ear to them. Sometimes characters come up with wonderful ideas to create twist and turns in the plot. Make your characters your friends.