Answer:
A minor character's storyline is as important and developed as the major characters' storylines.
Explanation:
Let's take a popular book series, like The Hunger Games or Harry Potter. In the Hunger Games, Katniss Evergreen is the main character, or, the major character as your question poses. The book focuses on her and her development. However, it also focuses on Peeta, who is technically a minor character, as the books predominantly focus on Katniss; but Peeta still had development, and proved to become vital to the storyline.
In Harry Potter, let's look at the 'Golden Trio'. Harry Potter is clearly the main character, and his development is evident all throughout the books. However, without his partners in crime, Ron Weasly and Hermione Granger, he never would have gotten very far. While there isn't much showcased in the books about their development, it's still there, just subtly so.
