“For anyone who wants to write good fiction, you need a character in there somewhere who creates tension and who is at odds with the forces of good.” -- Melissa Donovan in Fiction Writing Exercises: How to Write a Complex Villain.I don’t remember who said you should never write a story with only one character. I can’t think of any off hand that do. A few come to mind that have only one _active_ character. However, there are other people involved in the story, and one is the catalyst for the actions of the main character. I read a flash story recently in which a woman grumbles to herself about a few minor characters. She comes across as angry and bitchy, but the reader doesn’t know why until the end. It turns out she’s traveling to her father’s funeral and is having a difficult time accepting his death. So while the father isn’t an active participant in the story, he is that “other” character who causes the MC to act the way she does.
As for today’s quote, not only is the character needed to create tension, this person must be believable and should have some redeeming qualities. Otherwise, the reader won’t care what happens to either character.