Burt, this is a great question, because it brings up the topic of ACT STRUCTURE.
Without going into all the detail that I talk about in my online webinar and upcoming videos, let’s consider what Act 1 is. Act 1 is the beginning of the story. It contains everything the audience needs in order to understand Acts 2 and 3. Included in Act 1 are all the main characters and their initial relationships to one another.
In preparing this blog post I went looking for counter-examples. In THE MATRIX, the Oracle is introduced in Act 2. But of course, important though she is to the story, she is a supporting character rather than a main character. While her influence is felt throughout Act 2 and Act 3, she appears in only one scene.
In THE WIZARD OF OZ, Dorothy doesn’t meet the Wizard until Act 2—but again, though his influence is felt throughout Act 2 and Act 3, the Wizard appears in very few scenes. He is a supporting character rather than a main character.
How late in Act 1 can you introduce a main character? Any time—even at the very end. Again looking at THE MATRIX as an example, Morpheus isn’t introduced on screen until the end of Act 1.
By definition the beginning of your story—Act 1—isn’t over until all the main characters have been introduced.
To recap, your main characters and their initial relationships to one another should all be introduced in your Act 1, the beginning—roughly the first third—of your story.
I hope this has been helpful. Please ask me follow-up questions here or on Twitter.