A good question. When is the best time to introduce your main characters in your novel? I made a big mistake on this in my second novel—the draft did not feature the main character (the protagonist) and antagonist until later in Chapter 2 and Chapter 3. When I reworked the script to have these characters appear in Chapter 1, along with a strong hint of the conflict between them, it produced better tension and moved the plot along with more pace.
Definitions (from here):
The definition of a protagonist is basically “the main character”. Here is the Wikipedia definition:
“A protagonist (from Ancient Greek πρωταγωνιστής (protagonistes), meaning “player of the first part, chief actor”) is the main character in any story, such as a literary work or drama. The protagonist is at the center of the story, makes the key decisions, and experiences the consequences of those decisions.”
The Wikipedia definition of antagonist is the following:
An antagonist is a character whose motivations, goals, desires or opinions are opposed to those of the protagonist.
The important aspect in this alternative definition is the lack of hostility. Remember, I am in favor of tension to drive a story, but not in favor of (instant) trouble. Hope this helps your writing; it sure helped mine!