When editing manuscripts, the main thing I look at in characters is whether they are active or passive. An active character makes decisions and moves the story forward. Active characters change throughout the story, influenced by the decisions they've made. This progression of their character is called a "character arc."
A passive character is the opposite. Passive characters do not make many decisions to influence the story, and their character arcs are non-existent/very small. These characters tend to fall flat and can stall the story.
Your protagonist (the main character) will most likely be an active character. If they aren't an active character, chances are your story will be slow and won't have much progression. If your protagonist isn't making decisions to reach their goal, the reader will not have any reason to cheer them on during their journey. (This often results in readers putting the book down and moving on.) Strive for an active protagonist to keep your readers engaged.
It may seem like passive characters are bad, but they aren't--they have their own roles too. Passive characters tend to be the secondary characters. They can appear as a big part of the protagonist's life, or they can be a character that is only seen once or twice throughout the story.
Ultimately, you'll want to ensure your protagonist is an active character with a strong arc.